Useless errors for incorrect "threshold" keys

[kreczko]

Description

Currently, if wrong keys for thresholds are used you will see useless

Steps/code to reproduce issue

1. Pick config, e.g. config/rates.yaml
2. Change threshold entry JetET_BE to JetET
3. Run cmsl1t -c config/rates.yaml -n 100

Expected results

Error signalling that JetET_BE is not defined in thresholds

Actual results

Error: Please specify thresholds in the config .yaml in dictionary format
Traceback (most recent call last):
  File "/code/bin/cmsl1t", line 212, in <module>
    analyze()
  File "/software/cmsl1t/miniconda/envs/cms/lib/python2.7/site-packages/click/core.py", line 722, in __call__
    return self.main(*args, **kwargs)
  File "/software/cmsl1t/miniconda/envs/cms/lib/python2.7/site-packages/click/core.py", line 697, in main
    rv = self.invoke(ctx)
  File "/software/cmsl1t/miniconda/envs/cms/lib/python2.7/site-packages/click/core.py", line 895, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/software/cmsl1t/miniconda/envs/cms/lib/python2.7/site-packages/click/core.py", line 535, in invoke
    return callback(*args, **kwargs)
  File "/code/bin/cmsl1t", line 184, in analyze
    isok = run(config, nevents, reload_histograms)
  File "/code/bin/cmsl1t", line 128, in run
    process_tuples(config, nevents, analyzers, producers)
  File "/code/cmsl1t/utils/timers.py", line 13, in wrapper
    value = func(*args, **kwargs)
  File "/code/bin/cmsl1t", line 51, in process_tuples
    results = [analyzer.prepare_for_events(reader) for analyzer in analyzers]
  File "/code/cmsl1t/analyzers/HW_Emu_jetMet_rates.py", line 97, in prepare_for_events
    "L1 " + name, trig_thresholds, 16, 0, 80, ETA_RANGES.get(name))
  File "/code/cmsl1t/plotting/base.py", line 41, in build
    self.create_histograms(*vargs, **kwargs)
  File "/code/cmsl1t/plotting/rate_vs_pileup.py", line 25, in create_histograms
    self.thresholds = bn.GreaterThan(thresholds, "threshold", True)
  File "/code/cmsl1t/hist/binning.py", line 166, in __init__
    Base.__init__(self, len(bins), label,
TypeError: object of type 'NoneType' has no len()