ARM: fail less catastrophically on invalid Windows input

Windows supports a restricted set of relocations (compared to ARM ELF).  In some
cases, we may end up generating an unsupported relocation.  This can occur with
bad input to the assembler in particular (the frontend should never generate
code that cannot be compiled).  Generate an error rather than just aborting.

The change in the API is driven by the desire to provide a slightly more helpful
message for debugging purposes.

llvm-svn: 226779