Clean up messages and errors

There are some typos in a few strings, and a couple of strings which are pretty confusing.

Please create some nodereference fields for this content type.Please create some CCK Date fields for this content type OR enable the event module on it.

This needs a space between the sentences. Actually, they should be two seperate calls to form_set_error().

Please create or add nodereference fields to this content type.

Please create or add at least one CCK Date field for this content type <strong>or</strong> for the event module set this content type to "show in event calendar".

Fields to use as date for conflict checks:

Should be Field in the singular, since you can only select one field.

--Andrew