Better error messages

[jameysharp]

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.

[abrown]

Sounds good to me; better error messages in various places would be appreciated! Do you feel OK submitting a PR for this?