[llvm-exegesis] Add error for subprocess and dummy perf counters

When using the subprocess execution mode and dummy perf counters
currently, cryptic errors will be printed out related to not being able
to send the file descriptor through a socket. This patch prints out an
explicit message that this configuration isn't (currently) supported.

Reviewed By: courbet

Differential Revision: https://reviews.llvm.org/D157686