[clangd] Fix missing "message" key when responding with unsupported method

The language server protocol dictates that a ResponseError should have a
[message string][1] describing the error. This adds a simple message to the
error and a simple test.

[1]: https://github.com/Microsoft/language-server-protocol/blob/master/protocol.md#response-message

Patch by Raoul Wols!

Differential Revision: https://reviews.llvm.org/D38225

llvm-svn: 314119