Improve invalid test warnings/errors (#1325)

[beckjake]

Fixes #1325

Change the warning/error message for invalid test parameter types to look like this:

$ dbt test
Running with dbt=0.13.0
Compilation warning: Invalid test config given in models/schema.yml:
	test arguments must be dict, got <class 'list'> (value [{'column_name': 'id'}])
Found 4 models, 0 tests, 0 archives, 0 analyses, 172 macros, 4 operations, 0 seed files, 0 sources

The previous output looked like this:

$ dbt test
Running with dbt=0.13.0
in models/schema.yml: test arguments must be dict, got <class 'str'> (value abc)

[drewbanin]

LGTM!