Accidentally issuing -XX:StartFlightRecording for an image that do not include the jdk.jfr module will yield the following output:

[0.054s][error][jfr,system] jdk.jfr not found.
[0.055s][error][jfr,system] Flight Recorder can not be enabled.
[0.055s][error][jfr,system] To use Flight Recorder, you might need to add "--add-modules jdk.jfr" to the VM command-line options.

The "-add-modules" suggestion will not work on an image that does not include the actual module, so it should probably be removed from the output.

Also take the opportunity to clarify:

 "jdk.jfr not found" to "Module jdk.jfr not found" or something equivalent.