Fixed chromite shell to report python errors
Fixed chromite shell to report python errors

Errors in imported modules (where they are found but are broken) were being
reported as import errors. This fixes that behavior.

BUG=chromium-os:11785
TEST=try outside environment and make sure it detects error
try with a python error in main.py and check that it raises properly

Change-Id: Ic8a9fe75399832ed7c2faeb39360d5f9dc45e944

Review URL: http://codereview.chromium.org/6461009

bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,coreos/chromite,zhang0137/chromite,zhang0137/chromite,coreos/chromite,chadversary/chromiumos.chromite,zhang0137/chromite,coreos/chromite,bpsinc-native/src_third_party_chromite