Make TLC's missing config file error message clearer and more concise

When TLC is run without a configuration file, it prints the following error:

Error: TLC threw an unexpected exception.
This was probably caused by an error in the spec or model.
See the User Output or TLC Console for clues to what happened.
The exception was a tlc2.tool.ConfigFileException
: TLC encountered the following error when trying to read the configuration file DieHard.cfg:
File not found.

This message is overly verbose and misleading:

• It suggests there was an "unexpected exception," when in fact the issue is simply that the config file is missing.
• The text implies a problem with the spec or model, which isn’t accurate in this case.