Consolidate error messages and use common validation routines

[dhalbert]

I am (again) looking at consolidating error messages that are almost identical, and using common validation routines like the mp_arg_validate... routines when possible. The last time I tried this, it didn't save that much, but it does reduce the number of messages that need to be translated. I won't submit a final PR until after 7.3.x is branched, but I did a bunch already, and I have an older branch with similar work I need to look at again.