spirv-fuzz: consistently log cases where we give up

We should output a log message with a consistent format when we give up due to assumptions being violated (because we just wanted to handle the most common, simple cases). We should get gfauto to collect these messages so we remain aware of when we hit these in practice, and thus where we could improve spirv-fuzz by handling more complex scenarios.

Start by locating the spirv-fuzz paths that give up after assumptions are violated, then inspect how gfauto collects runtime messages. Define a consistent log format for these cases and verify that gfauto captures them so the occurrences remain visible in practice.

Written by the indexing model from the issue text.