genie_python: begin throws exception if RBNUMBER is unavailable

Seen on system tests - the begin command prints out the RB number for info, it looks like if this is not available begin will fail. BEGIN should not fail in these circumstances.

Acceptance criteria:
- absence of RB number pv does not cause begin to throw exception and so terminate script

** Beginning Run 00000493 at 04:49:31 19/05/19
Traceback (most recent call last):
File "C:\Instrument\Apps\Python\lib\site-packages\genie_python\genie.py", line 743, in begin
__api.dae.begin_run(period, meas_id, meas_type, meas_subid, sample_id, delayed, quiet, paused)
File "C:\Instrument\Apps\Python\lib\site-packages\genie_python\genie_dae.py", line 125, in _wrapper
return func(self, *args, **kwargs)
File "C:\Instrument\Apps\Python\lib\site-packages\genie_python\genie_dae.py", line 283, in begin_run
print(("* Proposal Number: {}".format(self.get_rb_number())))
File "C:\Instrument\Apps\Python\lib\site-packages\genie_python\genie_dae.py", line 664, in get_rb_number
return self._get_pv_value(self._get_dae_pv_name("rbnum"))
File "C:\Instrument\Apps\Python\lib\site-packages\genie_python\genie_dae.py", line 185, in _get_pv_value
return self.api.get_pv_value(name, to_string)
File "C:\Instrument\Apps\Python\lib\site-packages\genie_python\genie_epics_api.py", line 264, in get_pv_value
raise UnableToConnectToPVException(name, 'does not exist')
UnableToConnectToPVException: Unable to connect to PV TE:NDWRENO:ED:RBNUMBER: does not exist