Don't print two errors for unknown commands.

Summary:
We always print two error messages when we hit an unknown command. As the function
`CommandInterpreter::HandleCommand` that prints the second error message unconditionally called the `CommandInterpreter::ResolveCommandImpl` before (which prints the first error message), we can just remove
that second error message.

Fixes https://bugs.llvm.org/show_bug.cgi?id=38312

Reviewers: labath

Reviewed By: labath

Subscribers: labath, lldb-commits

Differential Revision: https://reviews.llvm.org/D49831

llvm-svn: 338040