CLC-4212 - Modify People addition placeholder and example error text in Canvas

Also fixes bugs reported in CLC-4128.

https://jira.ets.berkeley.edu/jira/browse/CLC-4128
https://jira.ets.berkeley.edu/jira/browse/CLC-4212

QA in #2908
@redconfetti looks like this string is copy/pasted - can you put it a level higher and re-use it below?