More verbose error on invalid Pythran spec

I stupidly put the argument names in my export and couldn't figure out my error :(

This commit explains the error better.

To see the difference, try ```#pythran export fac(int n)``` instead of ```#pythran export fac(int)``` in tests/rosetta/prime_decomposition.py for example.

Output will be something like
>E: Invalid Pythran spec near 'n' (line 26) 

Instead of
>E: Invalid Pythran spec