[PECOFF] Return an error_code instead of calling report_fatal_error().

For an invalid input we should not call report_fatal_error(), because
when LLD is used as a library, we don't want to kill the whole app
because of a malformed input.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@187673 91177308-0d34-0410-b5e6-96231b3b80d8