[llvm-exegesis] Add a custom error for clustering

[llvm-exegesis] Add a custom error for clustering

All errors of type `Failure` are `StringError`s. In order for exit code
mapping to detect that specifically a clustering error has occurred it
needs to have a different type.

This patch also prepares D74085 where termination `report_fatal_error()`
will be replaced with emitting `StringError`s.

Differential Revision: https://reviews.llvm.org/D74124