[llvm-exegesis] Improve error reporting

[llvm-exegesis] Improve error reporting

Fix inconsistencies in error reporting created by mixing
`report_fatal_error()` and `ExitOnErr()`, and add additional information
to the error message to make it more user friendly. Minimize the use
`report_fatal_error()` because it's meant for use in very rare cases and
it results in low information density of the error messages.

Summary of the new design:

 * For command line argument errors output `llvm-exegesis: <error_message>`,
   which is consistent with the error output format emitted by the backend
   which checks correctness of the command line arguments.
 * For other errors the format `llvm-exegesis error: <error_message>` is used.
 ** If the error occurred during file access `<error_message>` will have
    of two parts: `'<file_name>': <rest_of_the_error_message>`

Differential Revision: https://reviews.llvm.org/D74085