ENH - use %e instead of %f to report non-positive w2

otherwise it would just print non-informative 0 for small numbers above threshold

Author: yarikoptic

nibabel/quaternions.py

@@ -96,7 +96,7 @@ def fillpositive(xyz, w2_thresh=None):
     w2 = 1.0 - np.dot(xyz, xyz)
     if w2 < 0:
         if w2 < w2_thresh:
-            raise ValueError('w2 should be positive, but is %f' % w2)
+            raise ValueError('w2 should be positive, but is %e' % w2)
         w = 0
     else:
         w = np.sqrt(w2)