Improve error message for non-existent asset
Improve error message for non-existent asset

Now we print the name of the asset and the base directory where we looked for
the error instead of throwing a null pointer exception.

Fixes #296

cbracken/flutter,flutter/flutter,aghassemi/flutter,Hixie/flutter,Hixie/flutter,tvolkert/flutter,tvolkert/flutter,tvolkert/flutter,aghassemi/flutter,aghassemi/flutter,Hixie/flutter,tvolkert/flutter,Hixie/flutter,turnidge/flutter,flutter/flutter,flutter/flutter,cbracken/flutter,cbracken/flutter,Hixie/flutter,flutter/flutter,cbracken/flutter,tvolkert/flutter,turnidge/flutter,Hixie/flutter,tvolkert/flutter,turnidge/flutter,flutter/flutter,cbracken/flutter,turnidge/flutter,tvolkert/flutter,cbracken/flutter,Hixie/flutter,turnidge/flutter,Hixie/flutter,aghassemi/flutter,flutter/flutter,cbracken/flutter,flutter/flutter,cbracken/flutter,tvolkert/flutter,flutter/flutter,flutter/flutter,tvolkert/flutter,tvolkert/flutter,cbracken/flutter,cbracken/flutter,Hixie/flutter,Hixie/flutter,flutter/flutter