[dsymutil] Improve error reporting when we cannot create output file.

[dsymutil] Improve error reporting when we cannot create output file.

Before this patch we were returning an empty string in case we couldn't
create the output file. Now we return an expected string so we can
return and print the proper issue. We now return errors instead of bools
and defer printing to the call site.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@344983 91177308-0d34-0410-b5e6-96231b3b80d8