[m-rev.] for review: simplify options_file error handling

[Zoltan Somogyi]

For review by anyone.

Also, unless anyone objects, I intend to delete references
to erlang_flag in options_file.m, all the erlang-related options
in options.m, and all remaining erlang-related code in the
compiler, except the code that tells users that the erlang backend
is no more. That would of course be a separate commit.

Zoltan.
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