Fail if compiler discovery fails

[mattgodbolt]

If discovery fails for whatever reason (no compilers; compiler creation threw an exception) we now see this:

error: Exception while processing compilers: ...blah some thing that went wrong goes here...
error: #### No compilers found: no compilation will be done!
error: Top-level error (shutting down): No compilers found due to error or no configuration {"stack":"Error: No compilers found due to error or no configuration\n    at CompilerFinder.find (/home/matthew/dev/ce/compiler-explorer/lib/compiler-finder.js:408:19)\n    at processTicksAndRejections (node:internal/process/task_queues:96:5)\n    at async main (/home/matthew/dev/ce/compiler-explorer/app.js:476:36)"}

and the process exits.