Improve error messaging when there are issues with a (too) sparse marker set

[carmichaelong]

The web app returns a message if you attempt to upload a model file with fewer than 5 markers. There are a few ways use cases that lead to an immediate error without any messaging:

• You can upload a model with more than 5 markers from the original model (e.g., Rajagopal2015.osim), but none that match your data (motion_capture_walk.trc).
• You can upload a model (Rajagopal2015_exactset_only5.osim) with only 5 markers that are in your data.

While isolating the issue, here are some attempts from starting with the only the correct markers, but removing increasing numbers of markers (starting from the top of the list and going straight down):

• Works: Using the correct marker set but with one marker removed (Rajagopal2015_exactset_minus1.osim)
• Works: Removing all the markers on the pelvis, torso, and arms (Rajagopal2015_exactset_minus17.osim)
• Works: Removing all markers on pelvis, torso, and arms, + right leg thigh markers (Rajagopal2015_exactset_minus21.osim)
• Doesn't work (doesn't immediately fail as it gives some output to the logs, but no clear messaging for error): previous model as above but also remove right leg shank markers (Rajagopal2015_exactset_minus25.osim)

markerset-testing.zip