Catch EPERM and friends when running commands

[olorin]

Mar 24 07:07:35 baked boris-service: The supervised thread listener unexpectedly exploded: ./bin/ci.doc.branches: streamingProcess: runInteractiveProcess: exec
: permission denied (Permission denied)

The above was triggered by this build after I chmod -xed one of the build scripts; as the build thread dies with an exception the build status remains "pending" until it times out. We should catch this, fail the build and print an informative error message.

(/cc @NaevaTheCat, this is what happened to your support build)