lsp: Only mute RequestCancelled or ContentModified, but not other errors

handlers passed to `lsp_buf_request` weren't called if the server
responded with an error that looks like this:

     "decoded", {
      error = {
        code = -32601,
        message = "No delegateCommandHandler for foo"
      },
      id = 5,
      jsonrpc = "2.0"
    }

An example where that happens is both eclipse.jdt.ls and the
haskell-language-server when invoking a command that doesn't exist:

    :lua vim.lsp.buf_request(
      0,
      'workspace/executeCommand',
      { command = 'foo' },
      function(err, _, res)
        print(vim.inspect(err), vim.inspect(res))
      end
    )

0 files changed, 0 insertions, 0 deletions