Change "DataSet already present" message from Warning to Error

[hainm]

Hi, while working on Amber-MD/pytraj#1553, I've realized that cpptraj message is not very clear to me.

parm tz2.parm7
trajin tz2.nc
rms myrms @CA
radgyr myrms @CA nomax

Here is the message:

CPPTRAJ: Trajectory Analysis. V4.30.0 (GitHub)
    ___  ___  ___  ___
     | \/ | \/ | \/ | 
    _|_/\_|_/\_|_/\_|_

| Date/time: 10/11/20 22:11:19
| Available memory: 3.272 GB

INPUT: Reading input from 'c.in'
  [parm tz2.parm7]
	Reading 'tz2.parm7' as Amber Topology
	This Amber topology does not include atomic numbers.
	Assigning elements from atom masses/names.
	No SCEE section: setting Amber default (1.2)
	No SCNB section: setting Amber default (2.0)
  [trajin tz2.nc]
	Reading 'tz2.nc' as Amber NetCDF
  [rms myrms @CA]
    RMSD: (@CA), reference is first frame (@CA).
	Best-fit RMSD will be calculated, coords will be rotated and translated.
  [radgyr myrms @CA nomax]
Warning: DataSet 'myrms' already present.
Error: Could not initialize action [radgyr]
	1 errors encountered reading input.
TIME: Total execution time: 0.0671 seconds.
Error: Error(s) occurred during execution.

What's the reason of Error: Could not initialize action [radgyr]. Is that because of Warning: DataSet 'myrms' already present.. If yes, should the Warning should be Error?

[drroe]

What's the reason of Error: Could not initialize action [radgyr]. Is that because of Warning: DataSet 'myrms' already present.

Yes, you can't give 2 data sets the same name.

If yes, should the Warning should be Error?

Probably. I forget originally why I made it a warning. I think I had some half-baked idea to have cpptraj automatically append a number to the set so the calculation could continue, but ultimately decided that violated the principle of least surprise and wasn't worth it. I'll try to fix that this week.