Add check_parser_binary() validation method

This is mainly to prevent the user from selecting mediainfo as the subtitles parser if it has not yet been installed on the user's system somewhere in the PATH.

bazarr/app/config.py

@@ -7,6 +7,7 @@ import logging
 import re
 
 from urllib.parse import quote_plus
+from utilities.binaries import BinaryNotFound, get_binary
 from literals import EXIT_VALIDATION_ERROR
 from utilities.central import stop_bazarr
 from subliminal.cache import region
@@ -54,6 +55,14 @@ class Validator(OriginalValidator):
     )
 
 
+def check_parser_binary(value):
+    try:
+        get_binary(value)
+    except BinaryNotFound as e:
+        raise ValidationError(f"Executable '{value}' not found in search path. Please install before making this selection.")
+    return True
+
+
 validators = [
     # general section
     Validator('general.flask_secret_key', must_exist=True, default=hexlify(os.urandom(16)).decode(),
@@ -119,7 +128,7 @@ validators = [
     Validator('general.dont_notify_manual_actions', must_exist=True, default=False, is_type_of=bool),
     Validator('general.hi_extension', must_exist=True, default='hi', is_type_of=str, is_in=['hi', 'cc', 'sdh']),
     Validator('general.embedded_subtitles_parser', must_exist=True, default='ffprobe', is_type_of=str,
-              is_in=['ffprobe', 'mediainfo']),
+              is_in=['ffprobe', 'mediainfo'], condition=check_parser_binary),
     Validator('general.default_und_audio_lang', must_exist=True, default='', is_type_of=str),
     Validator('general.default_und_embedded_subtitles_lang', must_exist=True, default='', is_type_of=str),
     Validator('general.parse_embedded_audio_track', must_exist=True, default=False, is_type_of=bool),