iterating over the results of a missing macro gives a bad error

[beckjake]

Issue

Iterating over the results of a missing macro results in an unhelpful error

Issue description

Iterating over the result of a macro that's missing causes dbt to fail during parsing (eek) with a TypeError (double eek).

This is pretty common when users are using macros in an external package that return iterables (like dbt_utils.group_by()) and forget to run dbt deps.

The core problem is that our ParserMacroCapture override for Undefined during parsing has this:

def __call__(self, *args,**kwargs):
    return True

So any missing macros return True during parsing, and you can't iterate over that!

As a workaround, users can intuit that they need to run dbt deps, and then everything will work. But we should really make this error message less awful, and not require a deep understanding of jinja/dbt's internals to figure out what's happening.

Results

What I got:

$ dbt compile
Running with dbt=0.13.0
Encountered an error:
'bool' object is not iterable

What I expected:

Running with dbt=0.13.0
Found 1 models, 0 tests, 1 archives, 0 analyses, 101 macros, 0 operations, 0 seed files, 1 sources

07:50:51 | Concurrency: 2 threads (target='default')
07:50:51 |
Encountered an error:
Runtime Error
  Compilation Error in model x (models/x.sql)
    'missing_macro' is undefined

System information

dbt: Happens on 0.13.0, I assume most earlier versions as well.

OS/Python version agnostic.

Steps to reproduce

$ cat models/x.sql
{% for x in missing_macro() %}
{% endfor %}
$ dbt compile
Running with dbt=0.13.0
Encountered an error:
'bool' object is not iterable