input/source:

Refactored `nrml_to_hazardlib` to separate more cleanly conversion of sources
and ruptures.

Source conversion now catches conversion errors and raises a new exception
with the source id and name include, to help the user with debugging.
tests/input/source_test:

Added a test to exercise the case where `nrml_to_hazardlib` raises an error
and reports the src id and name.
Merge pull request #1128 from larsbutler/useful-source-model-err-msg

Source model NRML to hazardlib conversion now throws useful error messages

[r=matley] [f=1154512]