Change getReferenceInfo/getPairReferenceInfo to use new Error handling. NFC.

Adds a GenericError class to lld/Core which can carry a string.  This is
analygous to the dynamic_error we currently use in lld/Core.

Use this GenericError instead of make_dynamic_error_code.  Also, provide
an implemention of GenericError::convertToErrorCode which for now converts
it in to the dynamic_error_code we used to have.  This will go away once
all the APIs are converted.

llvm-svn: 264910