Resolves T11373. Currently Harbormaster gracefully handles a scenario where the build step implenementation is missing for a step. However, dependency calculations on valid build steps may cause them to attempt to access the implementations of other invalid build steps (in particular, when attempting to validate whether their input artifacts are available from other build steps).
When this happens, you get a blunt page crash like this:
Because of this page crash, there's no way to fix the invalid configuration from the web UI, and the user has to go and remove things from the database manually.
Instead, after this change, the page doesn't crash, but shows "Unable to calculate dependencies for this build step" when this issue occurs.