Catch visit_structure() Error more elegantly

[PatrickRWright]

Is your feature request related to a problem? Please describe.
visit_structure() only works for fixed visit plans. When it fails, the returned Error is cryptic.

Describe the solution you'd like
Since we can assume that the majority of fails in visit_structure() happens when the visit plan is not fixed we can catch the error and send a more helpful Error. e.g. "Likely your visit plan is too flexible to be plotted with this function."

Additional context
#155 (comment)

[PatrickRWright]

@aghaynes you want to do this before 1.0.0 or can I label is as after 1.0.0 for now? I don't think its a pressing issue.

[aghaynes]

It's probably be better to get it done before, don't you think? it's not clear at all at the moment...

[PatrickRWright]

@aghaynes do you have a timeline? I'd like to try and get the package on CRAN in April if possible.