Fix --warnings=+all:PATH not applying to nodoc.

Removed a special case treatment of warnings with PATH from the
implementation of +all warning.  This check only worked for the
Basic warning type but not for the All type, which throws no
exception when setting of warning has no effect.

Removed the "has no effect" exception from the Basic warning type,
as it wass just a little helper notice and not really crucial to
have.

Fixes #462