make test_forecast.py more robust, warn problems instead of fail

[wholmgren]

• Closes make test_forecast.py more robust #293
• I am familiar with the contributing guidelines.
• Fully tested. Added and/or modified tests to ensure correct behavior for all reasonable inputs. Tests (usually) must pass on the TravisCI and Appveyor testing services.
• Updates entries to docs/sphinx/source/api.rst for API changes.
• Adds description and name entries in the appropriate docs/sphinx/source/whatsnew file for all changes.
• Code quality and style is sufficient. Passes git diff upstream/master -u -- "*.py" | flake8 --diff
• New code is fully documented. Includes sphinx/numpydoc compliant docstrings and comments in the code where necessary.
• Pull request is nearly complete and ready for detailed review.

Brief description of the problem and proposed solution (if not already fully described in the issue linked to above):

This PR should resolve the majority of the intermittent test failures due to test_forecast.py. The errors will now be recorded as warnings. This is not ideal but I think it's acceptable.

I tested that all of the warnings are printed when I force exceptions in the try blocks.

[wholmgren]

Example of printed warning instead of a fail: https://travis-ci.org/pvlib/pvlib-python/jobs/412887886