Improve Error Messaging for Invalid Source Arguments

[elikastelein]

This pull request fixes #1660

This pull request enforces that exactly two arguments are provided to source() during compilation. If anything other than 2 arguments are given, a compiler error is now raised. This is an improvement over the current state described in #1660, where the error messaging is unclear after providing a single argument to source().

#1660 also mentions adding better error messaging in ref() but it already seems satisfactory to me: https://github.com/fishtown-analytics/dbt/blob/dev/louisa-may-alcott/core/dbt/context/parser.py#L98-L107

This is my first contribution to dbt so I will need help with the CI environment variables.

Thanks to @emilieschario for helping me with this.

[cla-bot]

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[drewbanin]

awesome, this looks great @CaptainEli!!

Are you able to sign the attached CLA? If so, we can definitely merge this for 0.15.0 :D

[drewbanin]

This looks pretty good to me -- going to kick off the CI suite now, but I think we'll me ready to merge this when the tests pass :)

@cla-bot check

[cla-bot]

The cla-bot has been summoned, and re-checked this pull request!

[emilieschario]

Great work @CaptainEli!!

[drewbanin]

ack - there's just one pep8 issue here:

core/dbt/context/parser.py:118:80: E501 line too long (84 > 79 characters)

If you're able to make that line <= 79 characters, then we should be ready to go here :)

[elikastelein]

@drewbanin thanks for the help. I've shortened it now 👍

[drewbanin]

Thanks @CaptainEli (a great username, to be sure)! This is great, merging it for the 0.15.0 release :)