So far using the "... from URL" or "... from an uploaded archive"
mechanism for installing a plugin would fail without further
information if the plugin to be installed was already installed.
The plugin manager will now detect this situation by parsing the
corresponding message from the pip output and trigger a reinstall
instantly.
A message about this will be logged to octoprint.log and the install
output.
Additionally the error handling for installation errors has been
slightly improved (install output no longer says "Done!" but "Error!"
with the reason as provided by the server) and the "could not install
plugin from URL unknown" issue should also be solved.
