Cleanup refactor of CCX utils

[pdpinch]

In reviewing https://github.com/edx/edx-platform/pull/10878 @singingwolfboy identified some issues that were out of scope for that PR. We'd like to get these addressed at some point.

• Raise an exception when validation fails instead of returning a boolean https://github.com/edx/edx-platform/pull/10878#discussion-diff-48857801
• Use dateutil instead of writing yet another date parser https://github.com/edx/edx-platform/pull/10878#discussion_r48857927
• first() instead of exists() https://github.com/edx/edx-platform/pull/10878#discussion_r48858136 (I'm not sure this will still be true)
• Having an action argument that controls what the function does is an anti-pattern https://github.com/edx/edx-platform/pull/10878#discussion_r48858607
• raise an exception instead of returning a list of errors https://github.com/edx/edx-platform/pull/10878#discussion_r48858836
• resetting the field cache smells bad https://github.com/edx/edx-platform/pull/10878#discussion_r48858992 (I have a feeling this is there for a reason)