More informative error msg when trying to "run" NeuroML2 files

[borismarin]

e.g.

$ ./jnml ~/src/NeuroML2/examples/NML2_SingleCompHHCell.nml
  jNeuroML v0.7.2
Loading: /Users/borismarin/src/NeuroML2/examples/NML2_SingleCompHHCell.nml with jLEMS...
org.lemsml.jlems.core.sim.ContentError: Cant read lems from XMLElt: neuroml, attrs: 4, body len: 0, children: 6
    at org.lemsml.jlems.core.reader.AbstractLemsFactory.buildLemsFromXMLElement(AbstractLemsFactory.java:29)
    at org.lemsml.jlems.core.sim.LemsProcess.readModel(LemsProcess.java:73)
    at org.neuroml.export.utils.Utils.readLemsNeuroMLFile(Utils.java:227)
    at org.neuroml.export.utils.Utils.loadLemsFile(Utils.java:342)
    at org.neuroml.export.utils.Utils.runLemsFile(Utils.java:336)
    at org.neuroml.JNeuroML.main(JNeuroML.java:270)
INFO Sep 25,2015 14:23  (INFO) Reading from: /Users/borismarin/src/NeuroML2/examples/NML2_SingleCompHHCell.nml