Print an error and exit when there are no plugins.

Issue: #1537

Running the :UpdateRemotePlugins command will show an ugly, unhelpful
error when no plugins were found. This change has neovim print an error message
and does not attempt to start the python-client which requires at least one
plugin.
Merge pull request #1539 from Floobits/master

Print an error and exit when there are no plugins.