trap confusing 'key error' related to missing time units

[andywood]

Some operations on input forcing files can leave them missing the proper time unit. If a file has 'seconds' instead of 'seconds since [some datum]', metsim fails complaining about a 'key error' (see below). It's fairly hard to figure out what's wrong, so trapping that particular error with a clear message would be useful.

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Traceback (most recent call last):
File "pandas/_libs/tslibs/timedeltas.pyx", line 528, in pandas._libs.tslibs.timedeltas.parse_timedelta_unit
KeyError: '/'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/glade/u/home/andywood/miniconda3/bin/ms", line 10, in
sys.exit(main())
File "/glade/u/home/andywood/miniconda3/lib/python3.6/site-packages/metsim/cli/ms.py", line 61, in main
ms = MetSim(setup)
File "/glade/u/home/andywood/miniconda3/lib/python3.6/site-packages/metsim/metsim.py", line 220, in init
period_ending=self.params['period_ending'])
File "/glade/u/home/andywood/miniconda3/lib/python3.6/site-packages/metsim/metsim.py", line 527, in _get_output_times
prototype = self.met_data
File "/glade/u/home/andywood/miniconda3/lib/python3.6/site-packages/metsim/metsim.py", line 291, in met_data
self._met_data = io.read_met_data(self.params, self.domain)
File "/glade/u/home/andywood/miniconda3/lib/python3.6/site-packages/metsim/io.py", line 182, in read_met_data
return process_funcs[params['forcing_fmt']](params, domain)
File "/glade/u/home/andywood/miniconda3/lib/python3.6/site-packages/metsim/io.py", line 210, in process_nc
calendar=params['calendar'], var_dict=params.get('forcing_vars', None))
File "/glade/u/home/andywood/miniconda3/lib/python3.6/site-packages/metsim/io.py", line 298, in read_netcdf
ds = ds.sel(time=slice(start, stop))
File "/glade/u/home/andywood/miniconda3/lib/python3.6/site-packages/xarray/core/dataset.py", line 2057, in sel
self, indexers=indexers, method=method, tolerance=tolerance
File "/glade/u/home/andywood/miniconda3/lib/python3.6/site-packages/xarray/core/coordinates.py", line 392, in remap_label_indexers
obj, v_indexers, method=method, tolerance=tolerance
File "/glade/u/home/andywood/miniconda3/lib/python3.6/site-packages/xarray/core/indexing.py", line 270, in remap_label_indexers
idxr, new_idx = convert_label_indexer(index, label, dim, method, tolerance)
File "/glade/u/home/andywood/miniconda3/lib/python3.6/site-packages/xarray/core/indexing.py", line 124, in convert_label_indexer
_sanitize_slice_element(label.step),
File "/glade/u/home/andywood/miniconda3/lib/python3.6/site-packages/pandas/core/indexes/base.py", line 4712, in slice_indexer
start_slice, end_slice = self.slice_locs(start, end, step=step, kind=kind)
File "/glade/u/home/andywood/miniconda3/lib/python3.6/site-packages/pandas/core/indexes/base.py", line 4925, in slice_locs
start_slice = self.get_slice_bound(start, "left", kind)
File "/glade/u/home/andywood/miniconda3/lib/python3.6/site-packages/pandas/core/indexes/base.py", line 4837, in get_slice_bound
label = self._maybe_cast_slice_bound(label, side, kind)
File "/glade/u/home/andywood/miniconda3/lib/python3.6/site-packages/pandas/core/indexes/timedeltas.py", line 321, in _maybe_cast_slice_bound
parsed = Timedelta(label)
File "pandas/_libs/tslibs/timedeltas.pyx", line 1217, in pandas._libs.tslibs.timedeltas.Timedelta.new
File "pandas/_libs/tslibs/timedeltas.pyx", line 371, in pandas._libs.tslibs.timedeltas.parse_timedelta_string
File "pandas/_libs/tslibs/timedeltas.pyx", line 509, in pandas._libs.tslibs.timedeltas.timedelta_from_spec
File "pandas/_libs/tslibs/timedeltas.pyx", line 530, in pandas._libs.tslibs.timedeltas.parse_timedelta_unit
ValueError: invalid unit abbreviation: /
distributed.process - WARNING - reaping stray process <ForkServerProcess(ForkServerProcess-15, started daemon)>
distributed.nanny - WARNING - Worker process 33682 was killed by unknown signal