improve error message when importing pandas from source directory

[mortada]

Currently if one imports from the source directory without building the C extension the error message is not very helpful:
ImportError: cannot import name 'hashtable'

I see a few previous PRs (#3827, #3821) that tried to address this but somehow this is still the behavior on master.

With this PR the error message would be a lot more informative:

ImportError: C extension: 'hashtable' not built. If you want to import pandas from the source directory, you may need to run 'python setup.py build_ext --inplace' to build the C extensions first.

[mortada]

@jreback @cpcloud please take a look

[jreback]

hmm, is it necessary to print the exception? I assume the interpreter will just quit in any event, not sure why was done that way previously. any thoughts?

[mortada]

if you mean the part about finding out which module causes the ImportError, yes it's not really necessary. But I suppose it was done to make the error message even more specific, i.e. "can't import hashtable, you need to build C extensions" vs "you need to build C extensions"

both would be much better than the current error that says "can't import hashtable" http://stackoverflow.com/questions/14422976/importing-pandas-shows-importerror-cannot-import-name-hashtable

[jreback]

looks ok to me. @shoyer @jorisvandenbossche @cpcloud
?

[shoyer]

Indeed, looks good to me

[jreback]

@mortada thanks!