Throw special exception when process' fns are called not in process context

[sskorokhodov]

It allows setting proper exit reason, especially in cases when process/exit is called with self pid as an argument.

[sskorokhodov]

process/exit finishes process with proper exit reason.
Throwing a special exception can be considered as a way to tell that process doesn't exist, but it isn't a reliable way to finish process function execution.