Print message when external command fails

Rather than raise an exception (which would suggest Nesta had crashed),
we print a short error message explaining what went wrong.

Fixes #128.
Continue parsing menu when a page not found.

Fixes gma/nesta#27.
Merge pull request #155 from jkowens/fix-27

Continue parsing menu when a page not found.
Use request.path_info when determining the selected page.

Fixes gma/nesta#112.