FEX sometimes fails silently

[ajax147]

Various conditions cause FEX to come up with the wrong answer (and it doesn't do or say what needs fixing):

1. Rx not set up correctly (FP return followed by EP return, then FocalPlane or whatever else continues the trace -- fail loudly)
2. Telecentric system (ludicrous EP placement -- best to accept what's already there)
3. EP beyond the FP (should be FP reference -- EP Return -- FP Return)
4. FP Return not really at FP location (take EP Return radius from iElt+1, not iElt-1)