errorlib: register_exception improvements

• Improves traceback analysis of register_exception by correctly employing inspect.trace API.

• Removes truncation of prefix and suffix variables set in the source code, when registering an exception, since these are often used to clearly put in evidence the data source that caused an error.

• Explicitly deletes inspected frames as recommended in http://docs.python.org/2/library/inspect.html#the-interpreter-stack.