Improve validations for derived metrics

[courtneyholcomb]

Resolves #270

Description

Improve validations for derived metrics.

• If there were no input metrics set, previously users would run into an AssertionError that interrupts the semantic manifest validation process. This changes that to be a more user-friendly validation error.
• Add a validation warning to ensure that expr is set and that it references all the input metrics.

Note: The first commit here was previously merged and then reverted because adding a ValidationError would be a breaking change. This PR updates it to be a ValidationWarning.

Checklist

• [x] I have read the contributing guide and understand what's expected of me
• [x] I have signed the CLA
• [x] This PR includes tests, or tests are not required/relevant for this PR
• [x] I have run changie new to create a changelog entry

[courtneyholcomb]

@WilliamDee mentioned in the PR description - ended up reverting that because it was a breaking change to add these as errors! Switched them to warnings here.