Fix E168: :finish used outside of a sourced file
Delete autocommands when Bootstrap failed

When bootstrapping fails to setup `g:_uspy` (e.g. without Python),
the functions used in the autocommands are not being defined.

Since it makes no sense to have autocommands defined for UltiSnips in this
case, this patch deletes them (via a new augroup 'UltiSnips').
Do not map keys if 'g:_uspy' could not be set
Add UltiSnipsFileType augroup and unset it if UltiSnips is disabled
Define all/other dummy functions used in autocommands
Do not barf more than once when python is not found. Patch by blueyed.