parsing of platforms_xxxx.conf does not catch all missing variables, error message not very descriptive

When creating a new experiment I made a small mistake in the MN3 platform description, SCRATCH_DIR was wrongly named CRATCH_DIR after editing.

"autosubmit create" ran fine, but "autosubmit run" gave a cryptic error "AttributeError: 'NoneType' object has no attribute 'endswith'"

First, this missing variable should probably be reported (as I think other variables such as USER are checked), and there should be a more user-friendly error reported.

etourign@bscesautosubmit01:/esnas/autosubmit/a0ch/tmp> autosubmit run a0ch
Preparing .lock file to avoid multiple instances with same expid.

Checking configuration files...
autosubmit_a0ch.conf OK
platforms_a0ch.conf OK
jobs_a0ch.conf OK
expdef_a0ch.conf  OK
Configuration files OK

[CRITICAL] Unhandled exception on Autosubmit: 'NoneType' object has no attribute 'endswith'
Traceback (most recent call last):
  File "/shared/earth/software/autosubmit/3.7.5-foss-2015a-Python-2.7.9/lib/python2.7/site-packages/autosubmit-3.7.5-py2.7.egg/autosubmit/autosubmit.py", line 324, in parse_args
    return Autosubmit.run_experiment(args.expid)
  File "/shared/earth/software/autosubmit/3.7.5-foss-2015a-Python-2.7.9/lib/python2.7/site-packages/autosubmit-3.7.5-py2.7.egg/autosubmit/autosubmit.py", line 602, in run_experiment
    submitter.load_platforms(as_conf)
  File "/shared/earth/software/autosubmit/3.7.5-foss-2015a-Python-2.7.9/lib/python2.7/site-packages/autosubmit-3.7.5-py2.7.egg/autosubmit/platforms/paramiko_submitter.py", line 129, in load_platforms
    remote_platform.user, remote_platform.expid)
  File "/shared/earth/software/Python/2.7.9-foss-2015a/lib/python2.7/posixpath.py", line 77, in join
    elif path == '' or path.endswith('/'):
AttributeError: 'NoneType' object has no attribute 'endswith'