Better message when we can't find a module

[JukkaL]

If mypy can't find a stub or implementation for a module, it says No module named 'x'. This can be pretty confusing for first-time users. Instead, we should say something like No implementation or stub file found for module 'x'.

Optionally, give a hint about MYPYPATH and the stub generator as well.

[JukkaL]

Now mypy tries to detect std library modules and 3rd party modules without stubs (at least the common ones) and gives a different error message based on the type of the module/package.