don't abort fgfs only because a nasal script called a property function

don't abort fgfs only because a nasal script called a property function
with an invalid path, as in getprop("/sim/model/737") or x.getNode("f:1").
Forward sg's error message to the Nasal runtime error function instead, so
you get something like:

  Nasal runtime error: name must begin with alpha or '_'
    at /home/m/fgfs/Base.local/Nasal/props.nas, line 30

Unfortunately, the location points to the line where the ghost wrapper
sits, rather than the offending script line.