[lldb/Utility] Introduce UnimplementedError

This is essentially a replacement for the PacketUnimplementedError
previously present in the gdb-remote server code.

The reason I am introducing a generic error is because I wanted the
native process classes to be able to signal that they do not support
some functionality. They could not use PacketUnimplementedError as they
are independent of a specific transport protocol. Putting the error
class in the the native process code was also not ideal because the
gdb-remote code is also used for lldb-server's platform mode, which does
not (should not) know how to debug individual processes.

I'm putting it under Utility, as I think it can be generally useful for
notifying about unsupported/unimplemented functionality (and in
particular, for programatically testing whether something is
unsupported).

Differential Revision: https://reviews.llvm.org/D89121