Add mechanism for generic error formatting to cli/args.zig

[mitchellh]

Extracted from #4444

The problem is that the error messages given by configurations are overly vague. The generic cli/args.zig parser only accepts a small error set of InvalidValue, InvalidFormat, etc. And all error messages are derived from that.

I propose introducing a mechanism by which the comptime type T to cli.args.parse can format its own error messages for an explicit error set E. This would all be statically typed and safe so we don't need anyerrors or implicit error sets or any of that madness.

See my comment in #4444 for what I'm thinking.