Improve lv2 effect builder error message

[SrMouraSilva]

Traceback (most recent call last):
  File "pedalpi.py", line 39, in <module>
    ep = builder.build('https://github.com/dcoredump/dexed.lv2')
  File "/usr/local/lib/python3.4/dist-packages/pluginsmanager/model/lv2/lv2_effect_builder.py", line 81, in build
    return Lv2Effect(self._plugins[lv2_uri])
KeyError: 'https://github.com/dcoredump/dexed.lv2'

This means I can't any information about the plugin {}. Try re-scan the installed plugins: builder.reload(builder.lv2_plugins_data())