Change all but the last ErrorOr<...> use for MachOUniversalBinary to Expected<...> to

allow a good error message to be produced.

I added the one test case that the object file tools could produce an error
message.  The other two errors can’t be triggered if the input file is passed
through sys::fs::identify_magic().  But the malformedError("bad magic number")
does get triggered by the logic in llvm-dsymutil when dealing with a normal
Mach-O file.  The other "File too small ..." error would take a logic error
currently to produce and is not tested for.

llvm-svn: 273946