Better error messages

Currently, the Rust JIT profiling API can only report an error with message "error when notifying event". That doesn't give me much to go on when troubleshooting, especially when all I have is this message and a stack trace reported by somebody else.

At minimum I think it would help if it called `iJIT_IsProfilingActive` and returned a different error message when loading the shared library failed, as opposed to something else going wrong.