[mlir-translate] More specific error message for BlockAddress

BlockAddress is currently unimplemented in the LLVM dialect of MLIR;
when converting to LLVM dialect MLIR from LLVM IR, mlir-translate
currently terminates with an "unhandled constant" error message.
Instead, this message could be made more specific, to let the user know
that the specific issue is that BlockAddress is unimplemented in the
LLVM dialect.

Differential Revision: https://reviews.llvm.org/D142337