feat: better graph validation errors

[psychedelicious]

Summary

• When validating edges, return the error message instead of just a boolean.
• When raising InvalidEdgeError, ensure we use that error message.
• Surface the error message to the user in the UI.
• Tweak the error toast to be a bit less cryptic.

Before:

After:

Related Issues / Discussions

Should help in troubleshooting the issue here https://discord.com/channels/1020123559063990373/1083864753543331981/1334888935242858506

QA Instructions

@skunkworxdark Could you please try reproducing that issue with this PR? Hopefully it's easier to see what's going on.

For this specific issue, might need to set breakpoints for each of the 3 fail case returns in these two methods in graph.py to get the full picture:

• Graph._is_iterator_connection_valid()
• Graph._is_collector_connection_valid()

Merge Plan

n/a

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)