Make parse error handling in BgpConfigParser less forgiving.

Note that BgpConfigParser should be treated as test code rather
than production code. Hence it's better to flag errors and fail
instead of ignoring them. Allows errors in tests to be detected
more easily.

Fix a couple of tests that had bad configuration.
Merge pull request #638 from Juniper/nsheth_config_parser

Make parse error handling in BgpConfigParser less forgiving.