Misc fixes - See desc

* Remove unnecessary uses of _list_from_options_callback
* Fix download tests - Bug from 6e84b21559
* Rename ExecAfterDownloadPP to ExecPP and refactor its tests
* Ensure _write_ytdl_file closes file handle on error - Potential fix for #517

yt_dlp/options.py

@@ -23,7 +23,7 @@ from .cookies import SUPPORTED_BROWSERS
 from .version import __version__
 
 from .downloader.external import list_external_downloaders
-from .postprocessor.ffmpeg import (
+from .postprocessor import (
     FFmpegExtractAudioPP,
     FFmpegSubtitlesConvertorPP,
     FFmpegThumbnailsConvertorPP,
@@ -803,9 +803,8 @@ def parseOpts(overrideArguments=None):
         action='store_true', dest='skip_download', default=False,
         help='Do not download the video but write all related files (Alias: --no-download)')
     verbosity.add_option(
-        '-O', '--print', metavar='TEMPLATE',
-        action='callback', dest='forceprint', type='str', default=[],
-        callback=_list_from_options_callback, callback_kwargs={'delim': None},
+        '-O', '--print',
+        metavar='TEMPLATE', action='append', dest='forceprint',
         help=(
             'Quiet, but print the given fields for each video. Simulate unless --no-simulate is used. '
             'Either a field name or same syntax as the output template can be used'))
@@ -1276,8 +1275,7 @@ def parseOpts(overrideArguments=None):
         help='Location of the ffmpeg binary; either the path to the binary or its containing directory')
     postproc.add_option(
         '--exec', metavar='CMD',
-        action='callback', dest='exec_cmd', default=[], type='str',
-        callback=_list_from_options_callback, callback_kwargs={'delim': None},
+        action='append', dest='exec_cmd',
         help=(
             'Execute a command on the file after downloading and post-processing. '
             'Same syntax as the output template can be used to pass any field as arguments to the command. '
@@ -1290,8 +1288,7 @@ def parseOpts(overrideArguments=None):
         help='Remove any previously defined --exec')
     postproc.add_option(
         '--exec-before-download', metavar='CMD',
-        action='callback', dest='exec_before_dl_cmd', default=[], type='str',
-        callback=_list_from_options_callback, callback_kwargs={'delim': None},
+        action='append', dest='exec_before_dl_cmd',
         help=(
             'Execute a command before the actual download. '
             'The syntax is the same as --exec but "filepath" is not available. '