caddyhttp: Improve listen addr error message for IPv6

[francislavoie]

Fixes an unclear message as per #4738

Old message:

validate: loading http app module: provision http: srv0: invalid listener address: :0

New message:

validate: loading http app module: provision http: srv0: invalid listener address: ::1:443, there are too many colons, so the port is ambiguous. Did you mean to wrap the IPv6 address with [] brackets?

Part of the issue is we were reusing the same variable before/after parsing, so addr would get clobbered with the empty NetworkAddress value, rendering it as :0. Not useful. I split up the variable name to fix that, and added a condition for when there's too many colons (kinda naive cause it doesn't look at whether [] was already there but whatever) to suggest a fix.

[mholt]

I'm often down for improving error messages. This seems useful, thanks!