Add error icon and handle pip install errors

[jaimergp]

Comes from item in #14. Report refers to this line:

napari-plugin-manager/napari_plugin_manager/qt_plugin_dialog.py

Line 59 in 7f90aa8

# TODO: add error icon and handle pip install errors

[jaimergp]

I think this is mostly an outdated comment because we do have some feedback methods to notify of errors now, so no need to have extra ones? Probably just needs to be deleted from the code to avoid confusion.