Added warning to nodes selector if nothing was matched

[Raalsky]

resolves #2115

Description

Added warning (or error when --warn-error provided) when resource selector fails due to lack of resources for example when the incorrect model name was provided.

Checklist

• I have signed the CLA
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• I have updated the CHANGELOG.md and added information about my change to the "dbt next" section.

[drewbanin]

Thanks for opening this PR @Raalsky! I dropped one copy request inline + adding @beckjake to review :)

[beckjake]

This will trigger a warning on any project that doesn't have sources (that's what the tests are failing about). I think this is a good feature, but I think we're going to have to change a bit more about how node selection works for this to function.

I think we could change get_selected and select_nodes to handle this case a bit more carefully: instead of calling coalesce in get_selected, do the replacement in select_nodes and preserve the initial value. If it's None, skip the check.

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[Raalsky]

Hi @beckjake. I've applied your suggestions and perform some refactors 😉 Feel free to reach out if you have any questions

[beckjake]

This looks good to me, one little spelling-related nit and I can kick off the full test suite.

[beckjake]

Looks good! I'll kick off the test suite and then we'll get this merged for 0.17.0