Report staging load errors

[Malcolmnixon]

This pull request implements #416 by adding code to report stage load errors by:

• Writing an error message to the log-file and console
• Breaking in the debugger (if present)
• Terminating the application with a non-zero exit code

Testing of a bad stage-load resulted in the following error output to the console:

ERROR: Cannot open file 'res://scenes/poke_demo/poke_demo_bad.tscn'.
   at: (scene/resources/resource_format_text.cpp:1659)
ERROR: Failed loading resource: res://scenes/poke_demo/poke_demo_bad.tscn. Make sure resources have been imported by opening the project in the editor at least once.
   at: (core/io/resource_loader.cpp:222)
ERROR: Error 2 loading resource res://scenes/poke_demo/poke_demo_bad.tscn
   at: push_error (core/variant/variant_utility.cpp:903)

The following was also written to the log file:

USER ERROR: Cannot open file 'res://scenes/poke_demo/poke_demo_bad.tscn'.
   at: load (scene/resources/resource_format_text.cpp:1659)
USER ERROR: Failed loading resource: res://scenes/poke_demo/poke_demo_bad.tscn. Make sure resources have been imported by opening the project in the editor at least once.
   at: _load (core/io/resource_loader.cpp:222)
USER ERROR: Error 2 loading resource res://scenes/poke_demo/poke_demo_bad.tscn
   at: push_error (core/variant/variant_utility.cpp:903)

[BastiaanOlij]

lgtm