more descriptive error message for 'could not get absolute uri'

[omccully]

What this PR does / why we need it:

I received this exception and it's not clear why. This will help people debug if they receive the same error.

Which issue(s) this PR fixes:

I did not make an issue for this (yet?)

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

No

Please make sure you've completed the relevant tasks for this PR, out of the following list:

• [x] Code compiles correctly
• [x] Created/updated tests
• [x] Unit tests passing
• [x] End-to-end tests passing
• [x] Extended the documentation
• [x] Provided sample for the feature

Thank you.

[omccully]

@dotnet-policy-service agree