[HltEfficiencyChecker]: Better error messages when annotated decay descriptor isnt specified properly.

Note: HltEfficiencyChecker is being moved from MooreAnalysis to DaVinci in !1005 (merged). This issue only becomes relevant to DaVinci once that is merged

Moved from MooreAnalysis#22 (closed). To be addressed soon after merge of !1005 (merged).

Reported by @mkenzie - many thanks.

If you don't add the annotations you'll get

File "/home/epp/phskbk/Scratch/lhcb/LHCbStack/stack/Moore/Hlt/Hlt2Conf/options/bnoc/eff_Bs2KpiKpi.py", line 231, in <module>
    run_moore_with_tuples(
  File "/home/epp/phskbk/Scratch/lhcb/LHCbStack/stack/MooreAnalysis/HltEfficiencyChecker/python/HltEfficiencyChecker/config.py", line 180, in run_moore_with_tuples
    top_cf_node = add_efficiency_tuples(
  File "/home/epp/phskbk/Scratch/lhcb/LHCbStack/stack/MooreAnalysis/HltEfficiencyChecker/python/HltEfficiencyChecker/config.py", line 279, in add_efficiency_tuples
    decay, branches, _ = parse_descriptor_template(descriptor_template)
  File "/home/epp/phskbk/Scratch/lhcb/LHCbStack/stack/MooreAnalysis/HltEfficiencyChecker/python/HltEfficiencyChecker/config.py", line 328, in parse_descriptor_template
    parent = particles[
IndexError: list index out of range

which isn't super helpful. Also, if you place the ${name} annotations inside the brackets rather than outside, you also won't get a helpful message.

These error messages should be improved.