Allow Contingent to supply a reason for non-conformance

When calling the ops framework, there are cases where nothing has a matching signature. But then there are cases where some op type signatures _did_ fully match, but the `conforms()` method returned `false`. Unfortunately, there is currently no way for an op to report _why_ it cannot work with particular arguments. We should add this, so that the matching service can give users better hints about why a particular op invocation cannot be fulfilled.

Start by tracing the ops framework's matching service and the conforms() method mentioned in the issue. Define how an op reports why it rejects otherwise matching arguments, then verify that the matching service can expose those reasons as user-facing hints; the issue names no files or tests to run.

Written by the indexing model from the issue text.