err -- print ProDOS error message

syntax:  err <integer>

ex:      err 70
         err $46

Prints the ProDOS error message corresponding to the given 
error code.  'err 0' prints a list of all the errors in the
table.

Davex never prints an error number if a message is available 
instead.  This command is useful if you get an error code 
from a program that doesn't have a table of ProDOS error 
messages built in.  (Applications created by 'sysalias' will
print two-digit error codes if something goes wrong; use
"err $xx" to see what they mean.)