utils.trim_string_for_EPICS() trimmed string too long

[prjemian]

The trimmed string should be trimmed thus: msg = msg[:MAX_EPICS_STRINGOUT_LENGTH-1]

We know this because:

usaxs@usaxscontrol /share1/USAXS_data $ caput 9idcLAX:USAXS:state  HDF5 file:/share1/USAXS_data/2019-04/04_
Old : 9idcLAX:USAXS:state            Running Flyscan
New : 9idcLAX:USAXS:state            HDF5 file:/share1/USAXS_data/2019-04/04
usaxs@usaxscontrol /share1/USAXS_data $ caput 9idcLAX:USAXS:state  HDF5 file:/share1/USAXS_data/2019-04/04x
Old : 9idcLAX:USAXS:state            HDF5 file:/share1/USAXS_data/2019-04/04
New : 9idcLAX:USAXS:state            HDF5 file:/share1/USAXS_data/2019-04/04
usaxs@usaxscontrol /share1/USAXS_data $ caput 9idcLAX:USAXS:state  12345678901234567890123456789012345678901234567890
Old : 9idcLAX:USAXS:state            Moving USAXS to USAXS mode
New : 9idcLAX:USAXS:state            123456789012345678901234567890123456789

discovered while running USAXS today:

DEBUG incoming msg=HDF5 file:/share1/USAXS_data/2019-04/04_19_test_usaxs/WC_530_0013.h5
DEBUG trimmed msg=HDF5 file:/share1/USAXS_data/2019-04/04_
Exception in thread Thread-787:
Traceback (most recent call last):
  File "/local/Apps/BlueSky/lib/python3.7/threading.py", line 917, in _bootstrap_inner
    self.run()
  File "/local/Apps/BlueSky/lib/python3.7/threading.py", line 865, in run
    self._target(*self._args, **self._kwargs)
  File "/home/beams11/USAXS/.ipython/profile_bluesky/startup/32-usaxs_flyscan.py", line 109, in prepare_HDF5_file
    user_data.set_state_blocking("HDF5 file:" + fname)
  File "/home/beams11/USAXS/.ipython/profile_bluesky/startup/10-devices.py", line 134, in set_state_blocking
    self.state.put(msg)
  File "/local/Apps/BlueSky/lib/python3.7/site-packages/ophyd/signal.py", line 900, in put
    self._write_pv.put(value, use_complete=use_complete, **kwargs)
  File "/local/Apps/BlueSky/lib/python3.7/site-packages/epics/pv.py", line 395, in put
    callback_data=callback_data)
  File "/local/Apps/BlueSky/lib/python3.7/site-packages/epics/ca.py", line 439, in wrapper
    return fcn(*args, **kwds)
  File "/local/Apps/BlueSky/lib/python3.7/site-packages/epics/ca.py", line 1443, in put
    PySEVCHK('put', ret)
  File "/local/Apps/BlueSky/lib/python3.7/site-packages/epics/ca.py", line 468, in PySEVCHK
    raise CASeverityException(func_name, message(status))
epics.ca.CASeverityException:  put returned 'Invalid string'