Use warn() instead of error() to report a bad symbol in a DSO.

Specifically, libwidevinecdm.so in Chrome has such bad symbol.
It seems the BFD linker handles them as local symbols, so instead
of inserting them to the symbol table, we should skip them too.

Differential Revision: https://reviews.llvm.org/D41257

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@320770 91177308-0d34-0410-b5e6-96231b3b80d8