BF: rudimentary docstring for mvpa2.datasets.sources

Detected thanks to
http://nipy.bic.berkeley.edu/builders/pymvpa-py2.7-osx-10.8/builds/1/steps/shell_3/logs/stdio
BF: process information is N/A on non-Linux, thus bad to rely on it for the baisc test of wtf()
BF: uglyish workaround for absent /status parsing on non-Linuxes
ENH: externals.exists -- allow to raise an arbitrary exception
BF: raise ImportError instead of RuntimeError if checked when importing module

I hope it would make nose react saner, see:
http://nipy.bic.berkeley.edu/builders/pymvpa-py2.7-osx-10.8/builds/1/steps/shell_3/logs/stdio
ENH: allow suite_stats get stats from any scope + test that we get entries of every type

for some strange reason on OSX buildbot datasets.source also became available
in a globals() which allowed to catch it missing a docstring.  It doesn't happen
on Linux'es and I was trying to achive the same failure by looking into the scope
of imported suite -- but could not replicate thus that code was left commented out