Let user function throw retry exception, instead of raw errors which pollute log #242

• Upgrade all tests to throw the retriable error where appropriate, to reduce error noise in test logs
• also include exception to skip the message

ATM, all exceptions thrown from user logs as ERROR and causes retry.

User should be able to catch exception, and rethrow a retriable exception which doesn't get logged by PC.

Be able to signify a retry to the system, without registering an ERROR - currently ERRORs must be thrown to make the system retry, but an actual RetriableException would make logs much better because we'll be able to distinguish between actual unexpected errors in user function execution, and retriable vs unretriable errors as decided by the user.