`bufexists` can't handle the home directory(`~`). Instead use `bufnr`
and `bufwinnr`, which are more appropriate.
Also, I've decided to use `bufwinnr` instead of `bufnr` for checking if
the file exists before loading it with `g:vebugger_view_source_cmd`, so
it'll use `g:vebugger_view_source_cmd` if no window displays the buffer
- even if the buffer is loaded. This fixes an inconsistency that was
before, that `g:vebugger_view_source_cmd` would get invoked only if the
file has no buffer loaded, if there is a buffer loaded it'll jump to it
from the current window.
fa63556530