[cli] Move output command state out of CliContext (#426)

* [cli] Move output command state out of CliContext

To simplify things, all output commands can be treated similarily
as they all act on the result of the processing pipeline. This
allows new commands to be added without needing to explicitly define
their values in CliContext, and makes the values that do remain there
much more meaningful.

It also allows commands to be specified multiple times gracefully
and with different options, so for example `save-images` can now be
run twice with different encoding parameters.

* [cli] Centralize preconditions in CliContext

* [cli] Simplify parsing of default values

scenedetect/__main__.py

@@ -22,10 +22,10 @@
 
 def main():
     """PySceneDetect command-line interface (CLI) entry point."""
-    cli_ctx = CliContext()
+    context = CliContext()
     try:
         # Process command line arguments and subcommands to initialize the context.
-        scenedetect.main(obj=cli_ctx)  # Parse CLI arguments with registered callbacks.
+        scenedetect.main(obj=context)  # Parse CLI arguments with registered callbacks.
     except SystemExit as exit:
         help_command = any(arg in sys.argv for arg in ["-h", "--help"])
         if help_command or exit.code != 0:
@@ -38,12 +38,12 @@ def main():
     # no progress bars get created, we instead create a fake context manager. This is done here
     # to avoid needing a separate context manager at each point a progress bar is created.
     log_redirect = (
-        FakeTqdmLoggingRedirect() if cli_ctx.quiet_mode else logging_redirect_tqdm(loggers=[logger])
+        FakeTqdmLoggingRedirect() if context.quiet_mode else logging_redirect_tqdm(loggers=[logger])
     )
 
     with log_redirect:
         try:
-            run_scenedetect(cli_ctx)
+            run_scenedetect(context)
         except KeyboardInterrupt:
             logger.info("Stopped.")
             if __debug__:
@@ -52,7 +52,7 @@ def main():
             if __debug__:
                 raise
             else:
-                logger.critical("Unhandled exception:", exc_info=ex)
+                logger.critical("ERROR: Unhandled exception:", exc_info=ex)
                 raise SystemExit(1) from None