[llvm-exegesis] Improve error reporting in BenchmarkRunner.cpp

Followup to D74085.
Replace the use of `report_fatal_error()` with returning the error to
`llvm-exegesis.cpp` and handling it there.
To facilitate this, a new `Error` type has been added which is only used
to log errors to the yaml output.

Differential Revision: https://reviews.llvm.org/D74215