[DebugInfo] Add error handling to DWARFDebugAbbrev::getAbbreviationDeclarationSet

This gives us more meaningful information when
`getAbbreviationDeclarationSet` fails. Right now only
`verifyAbbrevSection` actually uses the error that it returns, but the
other call sites could be rewritten to take advantage of the returned error.

Differential Revision: https://reviews.llvm.org/D153459