throw a better error when a user tries to archive a nonexistent table

[drewbanin]

Issue

Issue description

If a user tries to archive a non-existent table, dbt shows an error like:

Relation None not found!

Instead, this should show the name of the relation which was not found. It should also contextualize that this error occurred during archival.

Results

Saw:

Relation None not found!

Would like:

Error while running archive:
The relation to archive, `{schema}.{table}` was not found! Check that the relation exists, and that your user has permissions to select from this relation,

System information

The output of dbt --version:

0.11.x

[beckjake]

A fun side effect of the new archive blocks feature is that this is magically fixed! 🎉

Because archives are now queries instead of configured, dbt just takes the raw query and tries to perform it, and the database gives you a pretty helpful error (well, I think so!):

Completed with 1 errors:

Database Error in archive foo (archives/test.sql)
  relation "not_a_schema.not_a_table" does not exist
  LINE 8:     select * from not_a_schema.not_a_table where status = 'c...