Rectangular export gives missleading Error indication

[PatrickRWright]

Describe the bug
If the export is rectangular, it throws an error “Input to read_secuTrial() appears to be incompatible. Have you exported CSV format?”. This might be misleading as this export is CSV but in rectangular format…just in case people won’t read the export requirements…and I saw that actually this error message should appear:

if (export_options$is_rectangular) {

stop("Your export is rectangular. It can not be loaded with this function.")

}

Expected behavior
Report rectangular structure as the actual problem.

Additional context
#155 (comment)