Include original path in CommandNotFound.
Include original path in CommandNotFound.

Whenever `which()` fails to resolve the path it returns `None` and
overwrites the original `path` value. Later when we create 
`CommandNotFound` exception we always pass `None` instead of the
original `path` value and it makes `CommandNotFound` not so usefull.
amoffat/sh,simudream/sh,twocngdagz/sh,pdelong42/sh,germangh/sh,pombredanne/sh,achernet/sh,lwgarmstrong/sh,amoffat/sh,z-fork/sh,Stupeflix/sh,rdhyee/sh,ppannuto/sh,rsmith31415/sh,jakirkham/sh