This PR proposes to rephrase the warning for non-exhaustive patterns to make it clearer that counter-examples printed by the warning are patterns or cases and not values (that could be constructed).

For instance, with the current warning message, the following code

type t = { left:int ; right:int }
let f = function { left = 0; _ } -> ();;

raises a warning

Warning 8: this pattern-matching is not exhaustive.
Here is an example of a value that is not matched:                                     
{left=1}

This warning is confusing for beginners on at least two fronts. First, {left=1} is not a valid ocaml value. A beginner trying to see what happens when computing f {left=1} may be lost in the error raised by the compiler. The counter-example {left=1} is a pattern and should not be introduced as a value. Rewording the warning message as

Warning 8: this pattern-matching is not exhaustive.
Here is an example of a case that is not matched:                                     
{left=1}

solves this ambiguity without decreasing the information density of the warning.
This is the object of the first commit in this PR.

Second, the pattern {left=1} elides the field right without any indication that fields have been elided. Adding a trailing ; _ when fields have been elided would correct this lack of information and reinforces the notion that the counter-example is a pattern:

Warning 8: this pattern-matching is not exhaustive.
Here is an example of a value that is not matched:                                     
{left=1; _ }

The second commit in this PR modifies the pattern pretty printer in typing\parmatch.ml
to add this trailing _ when record patterns are not closed.

See also mantis:7169 for more discussions.