Improve exception info in s:SetRoot

Ref: https://github.com/tomtom/tplugin_vim/issues/11
Handle optional `plugin` via `file` in `s:DefineCommand`

Fixes https://github.com/tomtom/tplugin_vim/issues/11
Merge branch 'better-error' of https://github.com/blueyed/tplugin_vim
FIX #11: Also remove "general" autoloads ("*")