Collect request errors into req.errors object.

And add a cause property when the middleware processing was aborted or
bailed.

The req.errors object has a *list* property containing all errors
encountered (except internal cause errors) and a *has()* function to
determine if any errors occurred during processing.

Useful if you need to test for an error condition in a complete listener
or parse callback.
0.8.210
Remove debug statement.
0.8.211
Support ${see_*} variables in substitution.

Provides variables for top-level commands such that $see_all is a comma
delimited list of all top-level commands and $see_cmd for a program
named *prg* will equal `prg-cmd(1)`.
0.8.212