Clarify pkg_export error message.
Clarify pkg_export error message.

This makes the error message super clear that the value must be a valid
TCP or UDP port, and what the value it found was instead.

Signed-off-by: Ryan Hass <7a1efea0847aa10fda44596a4a622bd3187d968e@users.noreply.github.com>

nathenharvey/habitat,habitat-sh/habitat,georgemarshall/habitat,georgemarshall/habitat,rsertelon/habitat,nathenharvey/habitat,habitat-sh/habitat,georgemarshall/habitat,habitat-sh/habitat,georgemarshall/habitat,georgemarshall/habitat,rsertelon/habitat,nathenharvey/habitat,habitat-sh/habitat,nathenharvey/habitat,georgemarshall/habitat,habitat-sh/habitat,georgemarshall/habitat,nathenharvey/habitat,habitat-sh/habitat,rsertelon/habitat,nathenharvey/habitat,habitat-sh/habitat,rsertelon/habitat,georgemarshall/habitat,rsertelon/habitat,habitat-sh/habitat,rsertelon/habitat,rsertelon/habitat