Throw tool exit on malformed yaml in flutter manifest

The `loadYaml` call here:

https://github.com/flutter/flutter/blob/master/packages/flutter_tools/lib/src/flutter_manifest.dart#L38

can thrown an exception when the yaml is invalid. We should catch it and `throwToolExit` instead with a helpful error message.

So far this is 36.3% of tool crashes in 1.7.8+hotfix.3. This crash was not present in the last stable release.