[lldb] Improve error reporting in `lang objc tagged-pointer info`

Improve error handling for the lang objc tagged-pointer info. Rather
than failing silently, report an error if we couldn't convert an
argument to an address or resolve the class descriptor.

  (lldb) lang objc tagged-pointer info 0xbb6404c47a587764
  error: could not get class descriptor for 0xbb6404c47a587764

  (lldb) lang objc tagged-pointer info n1
  error: could not convert 'n1' to a valid address

Differential revision: https://reviews.llvm.org/D112945