calc:

Calculators which loop over sources now include a try/except inside
the source loop so that the source_id of the source can be propagated
to the user (in an error message) in the case of a failure.
tests/calc/hazard_curve_test:

Factored out test fixtures for `HazardCurvesTestCase` into a `setUp`
method.
tests/calcs/hazard_curve_test:

Added a test to exercise the functionality where an error occurs with a
source. This test ensures that the error is captured and re-raised with
the source_id included in the error message.
tests/calcs/hazard_curve_test:

Whitespace cleanup.
tests/calc/hazard_curve_test:

Removed a redundant assert.
tests/calc/disagg_test:

Added `source_id` attributes to FakeSource test objects.
tests/calcs/disagg_test:

Added `test_source_errors` to test for handling/source_id reporting in
the case of computation failures on a given source.
tests/calc/stochastic_test:

Added `source_id` attribute to FakeSource objects, and factored out the
test fixtures into a `setUp` method.
tests/calc/stochastic_test:

Added a `test_source_errors` test case.
tests/calc/disagg_test:

Whitespace cleanup.
Merge pull request #120 from larsbutler/calcs-report-source-id-on-failure

Calculators report source_id on failure

[r=micheles] [f=1174207]