lit: warn when passed invalid pathname

It would previously say things like

  warning: input 'test/Frontend/foo.c' contained no tests

and have the user pull their hair trying to figure out what's wrong with that
file. This patch changes the message to the much clearer:

  warning: no such file or directory: 'test/Frontend/foo.c'

Differential Revision: http://reviews.llvm.org/D4097