Fail on parse errors (#1493)

[beckjake]

Fix #1493

• Raise exceptions where we used to (usually) just warn, kind of --strict all the time
• Fix some error messages to be a bit more informative (though you still kind of have to piece stuff together on large, deeply nested validation errors)
• Hoisted logging about those errors up into the GraphLoader itself, which then re-raises exceptions to ensure dbt actually stops. This is a bit more useful, I think, given that we are failing immediately in all those cases now.
• We had some tests that asserted the opposite of what this PR accomplishes, so I've reversed those accordingly

[drewbanin]

This looks great! Ship it :)