Let haskell-process-hoogle-ident fail silently on error

Small change to let haskell-process-hoogle-ident fail silently if the call to hoogle has a nonzero exit code.

In that case the output won't be valid anyway, however currently the error message is parsed and suggested as the module name to import. (E.g. 'Unknown option --exact' leads to 'import Unknown')

I don't know if failing silently is optimal in this case but at least it does not give wrong results :)

Best,
Markus