Exit with fatal error if local manifest file cannot be parsed

If the .repo/local_manifests folder includes a local manifest file
that cannot be parsed, the current behaviour is to catch the parse
exception, print a warning, and continue to process remaining files.

This can cause any errors to go unnoticed.

Remove the exception handling, so that the exception is instead
caught in main._Main, and repo exits with a fatal error.

Change-Id: I75a70b7b850d2eb3e4ac99d435a4568ff598b7f4