Add ApiError class to camp-server.

This class is a straight copy of brooklyn-rest ApiError, which was moved
from brooklyn-utils-rest-swagger. It should eventually be replaced by an
ErrorMessage object.
Merge pull request #4 from sjcorbett/api-error

Adds ApiError class to camp-server.
Merge branch 'master' into feature/pdp

Conflicts:
	camp-server/src/main/java/io/brooklyn/camp/rest/util/WebResourceUtils.java

Replace ApiError (exact copy) with ApiErrorDto from sam (better comments and location)