Fix plot crash on negative errors

[jhaigh0]

Description of work

Fix crash in the case of plotting data with negative errors.

Instead, print a warning and turn the negative errors positive.

Fixes #37973

To test:

• Load this data with negative errors
  BCA299_Subt_QLd_Result.zip

• Plot Spectra with errors 6-8

• Should see a warning printed but plot opens normally.

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