[fidlc] Check for errors when parsing CompoundIdentifiers

Prior to this, we parsed the first Identifier in a CompoundIdentifier
and access it immediately. This fails if the first Identifier fails to
parse.

Test: new test case in fidl-compiler host test

FIDl-263 #done

Change-Id: I7f39a123349ec34c6a25d43fb2eff236fba8ff9e