Question:
How can I show a list of constraint error messages to the
user on the client, correct the errors and re-post at one
time?
Answer:
You can only ever have one failed constraint at a time.
For example, if you have 3 record level constraints and
they are all going to fail when you post, then you will
get an exception when you post that corresponds to the
first constraint that failed. From there, you can't get
the second constraint that failed until you fix the problem
with the first one and attempt to re-post.