[FEATURE] New parameter for required subtitle languages, and fixes (#1357)

Closes https://github.com/jmbannon/ytdl-sub/issues/1269

Adds the new paramter `languages_required`, which forces an entry to have this set of subtitle files, otherwise it will error.
In addition, better file checking has been added to ensure they exist before attempting to move.

docs/source/config_reference/plugins.rst

@@ -980,6 +980,14 @@ It will set the respective language to the correct subtitle file.
   language codes. Defaults to only "en".
 
 
+``languages_required``
+
+:expected type: Optional[List[String]]
+:description:
+  Language code(s) that are required to be present for downloads to continue. If missing,
+  ytdl-sub will throw an error. NOTE: currently this only checks file-based subtitles.
+
+
 ``subtitles_name``
 
 :expected type: Optional[EntryFormatter]

src/ytdl_sub/plugins/subtitles.py

@@ -1,8 +1,10 @@
+import os
 from pathlib import Path
 from typing import Dict
 from typing import List
 from typing import Optional
 from typing import Set
+from typing import Tuple
 
 from ytdl_sub.config.plugin.plugin import Plugin
 from ytdl_sub.config.plugin.plugin_operation import PluginOperation
@@ -11,6 +13,7 @@ from ytdl_sub.downloaders.ytdl_options_builder import YTDLOptionsBuilder
 from ytdl_sub.entries.entry import Entry
 from ytdl_sub.entries.script.variable_definitions import VARIABLES
 from ytdl_sub.entries.script.variable_definitions import VariableDefinitions
+from ytdl_sub.script.utils.exceptions import UserThrownRuntimeError
 from ytdl_sub.utils.file_handler import FileHandler
 from ytdl_sub.utils.file_handler import FileMetadata
 from ytdl_sub.utils.logger import Logger
@@ -57,6 +60,7 @@ class SubtitleOptions(ToggleableOptionsDictValidator):
         "subtitles_type",
         "embed_subtitles",
         "languages",
+        "languages_required",
         "allow_auto_generated_subtitles",
     }
 
@@ -76,6 +80,9 @@ class SubtitleOptions(ToggleableOptionsDictValidator):
         self._languages = self._validate_key_if_present(
             key="languages", validator=StringListValidator, default=["en"]
         ).list
+        self._languages_required = self._validate_key_if_present(
+            key="languages_required", validator=StringListValidator, default=[]
+        ).list
         self._allow_auto_generated_subtitles = self._validate_key_if_present(
             key="allow_auto_generated_subtitles", validator=BoolValidator, default=False
         ).value
@@ -121,6 +128,16 @@ class SubtitleOptions(ToggleableOptionsDictValidator):
         """
         return [lang.value for lang in self._languages]
 
+    @property
+    def languages_required(self) -> Optional[List[str]]:
+        """
+        :expected type: Optional[List[String]]
+        :description:
+          Language code(s) that are required to be present for downloads to continue. If missing,
+          ytdl-sub will throw an error. NOTE: currently this only checks file-based subtitles.
+        """
+        return [lang.value for lang in self._languages_required]
+
     @property
     def allow_auto_generated_subtitles(self) -> Optional[bool]:
         """
@@ -211,7 +228,21 @@ class SubtitlesPlugin(Plugin[SubtitleOptions]):
         if self.plugin_options.embed_subtitles:
             file_metadata = FileMetadata(f"Embedded subtitles with lang(s) {', '.join(langs)}")
         if self.plugin_options.subtitles_name:
+            download_subtitle_lang_file_names: List[Tuple[str, str]] = []
+
             for lang in langs:
+                download_subtitle_file_name = entry.base_filename(
+                    ext=f"{lang}.{self.plugin_options.subtitles_type}"
+                )
+                if os.path.isfile(Path(self.working_directory) / download_subtitle_file_name):
+                    download_subtitle_lang_file_names.append((lang, download_subtitle_file_name))
+                elif lang in self.plugin_options.languages_required:
+                    raise UserThrownRuntimeError(
+                        f"Required the subtitle lang {lang}, but the file could not be found for "
+                        f"the entry {entry.title}."
+                    )
+
+            for lang, file_name in download_subtitle_lang_file_names:
                 output_subtitle_file_name = self.overrides.apply_formatter(
                     formatter=self.plugin_options.subtitles_name,
                     entry=entry,
@@ -219,9 +250,7 @@ class SubtitlesPlugin(Plugin[SubtitleOptions]):
                 )
 
                 self.save_file(
-                    file_name=entry.base_filename(
-                        ext=f"{lang}.{self.plugin_options.subtitles_type}"
-                    ),
+                    file_name=file_name,
                     output_file_name=output_subtitle_file_name,
                     entry=entry,
                 )