Missing vars in generate_schema_name macro do not fail the run

[beckjake]

Describe the bug

When there is a var('foo') call in a generate_schema_name macro, if foo does not exist, dbt returns None for it.

Steps To Reproduce

1. Add this macro:

{% macro generate_schema_name(custom_schema_name, node) -%}
    {% do log(' *** The var is: ' ~ var('myvar'), info=true) %}
    {% do return(target.schema) %}
{%- endmacro %}

2. Do not declare myvar
3. Run dbt
4. get a bunch of log messages that The var is: None.

Expected behavior

dbt should fail with an error about the var not being set.

System information

Any database, any python, any OS.

The output of dbt --version:

0.17.0rc1

Additional context

This is caused by dbt using the parse context for the generate_*_name macros. Really, we want a special context that is mostly the parse context except with runtime behavior for var.

This fix should be fairly straightforward as the context is only used for these macros, we can just change the macros we call.

[drewbanin]

closed by #2477