edge case bug

fixed a bug preventing generation of helpful error messages in response to non-sensical user input (eg "@atom:"). The previous version would crash with an uncaught exception when presented with this input:

write_once("In Settings") {
  pair_coeff @atom:WallParticle/W @atom: 
}
Molecule {
  write("Data Atoms") {
    $atom:w $mol:. @atom:W  0.0  0.0 0.0 0.0
  }
}

(The syntax error in this example is on line 2. The new version of ttree.py and moltemplate.sh generate error messages instead of crashing.)