Catch name_mapper exceptions

[gabelevi]

You can write name mappers like

module.name_mapper='foo\(bar\)?' -> '\1'

which will error since the matching group might not exist. This leads to an uncaught exception. This should be easy to fix - we should wrap the Str.global_replace calls in https://github.com/facebook/flow/blob/master/src/services/inference/module_js.ml with try/with blocks, emit an error, and soldier on.

Thanks @billschaller for the report!