Warning should be surfaced if any part of the --models specification doesn't exist

[kingfink]

Describe the bug

A clear and concise description of what the bug is. What command did you run? What happened?

Assume model_a exists and model_b does not exist in a given dbt project.

If you run dbt run --models model_b you get an error: WARNING: Nothing to do....

If you run dbt run --models model_a model_b you don't get any error since one of the models exists.

It seems like it would make sense to surface an error like WARNING: Model specification matching model_b not found or something like that

Steps To Reproduce

In as much detail as possible, please provide steps to reproduce the issue. Sample data that triggers the issue, example model code, etc is all very helpful here.

See above

Expected behavior

A clear and concise description of what you expected to happen.

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Screenshots and log output

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System information

Which database are you using dbt with?

• postgres
• redshift
• bigquery
• snowflake
• other (specify: ____________)

The output of dbt --version:

Running with dbt=0.15.0

The operating system you're using:

OSX

The output of python --version:

Python 3.7.4

Additional context

Add any other context about the problem here.

Definitely not an urgent problem, but figure it's a good one to mention

[drewbanin]

Thanks for the report @kingfink - this is a good idea! We should make this a highly visible warning that can be raised to an error using the --warn-error flag

[jeremyyeo]

@drewbanin seems like this is somewhat related to #1765?

[drewbanin]

ha! Thanks @jeremyyeo - I was looking for that one! Just closed #1765 in favor of this issue. Appreciate you digging for it :)