resolve: report duplicate module definitions

add_child() is responsible for reporting errors about type, value, and
module duplicate definitions.  Although it checks for all three, it uses
namespace_to_str() to convert a Namespace value into a string before
printing an error like:

  error: duplicate definition of type `foo`
                                 ^^^^
  note: first definition of type foo here:
                            ^^^^

Unfortunately, this string can only be one of "type" or
"value" (corresponding to TypeNS and ValueNS respectively), and it
reports duplicate modules as duplicate types.

To alleviate the problem, define a special NamespaceError enum to define
more specialized errors, and use it instead of attempting to reuse the
Namespace enum.

Reported-by: Corey Richardson <corey@octayn.net>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
resolve: make error message strings more consistent

The "first definition of ..." error string reported by add_child() looks
different from similar messages reported by other functions.  Fix this.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>