Improve error messages for plot_foi_estimates and plot_rhats

[jpavlich]

When passing constant seromodel as parameter to plot_foi_estimates and plot_rhats, it yields the following error

Error in plot_foi_estimates(seromodel, serosurvey) : 
  seromodel@name should start with either 'age' or 'time'

It is a little unintuitive, since it assumes the user know that seromodel@model_name begins with 'age' or 'time' for age or time models, respectively.

I would suggest changing it to "seromodel type must be 'age' or 'time'"

[jpavlich]

@ntorresd If you agree, you can assign me this issue. I will fix it as soon as #215 is merged into dev

[ntorresd]

#223 has been merged to dev. Please proceed with this @jpavlich.