Provide better feedback when there's an intervention type error

[nt-williams]

Capture this error message and indicate setting intervention_type = "mtp" will likely fix it.

Error in glm.fit(x = c(1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,  :            
  object 'fit' not found