Change Calibration error message

[Elizabeth Brainerd]

"Control your data" is not helpful. If users have multiple pairs of images, message could say:

Error: Reset Frame and try a different set of images.

If just one:

Error: Reset Frame and try again.

[Benjamin Knorlein]

I changed it to: Please try to select the references again. If you keep getting this error please have a look at the 3D view and check if the already computed cameras are correct. If necessary reset the erroneous frames or reset the camera!

[Benjamin Knorlein]

fixes #87 #88 : Increase Thresholds for marker detection and changed calibration error text.

→ <<cset 253d9fd398b4>>