5ttgen: Fix erroneous warning
5ttgen: Fix erroneous warning

In 63f51aad, the 5ttcheck command was altered to print a console message if all input images check out OK. This however conflicts with the 5ttgen script, which used non-empty 5ttcheck stderr contents to trigger a warning message. This commit alters 5ttgen to examine the 5ttcheck output contents more closely, only echoing any warnings if any were actually issued.

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3