Add Pattern Match On Exceptions as a reasonml til

reason/pattern-match-on-exceptions.md

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+# Pattern Match On Exceptions
+
+[ReasonML](https://reasonml.github.io/) supports [powerful pattern
+matching](https://reasonml.github.io/docs/en/pattern-matching.html) through
+the `switch` statement.  This even includes pattern matching against
+exceptions that may arise as a way of catching and handling those
+exceptions.
+
+```reason
+let values: list(int) = [1,3,5,7,9];
+
+let getValue = (list, index) => {
+  switch (List.nth(values, index)) {
+  | value => value
+  | exception Failure("nth") => 0
+  | exception Invalid_argument("List.nth") => 0
+  };
+};
+
+getValue(values, 0); /* 1 */
+getValue(values, 1); /* 3 */
+getValue(values, 5); /* 0 */
+getValue(values, -1); /* 0 */
+```
+
+The [`List.nth` docs](https://reasonml.github.io/api/List.html) detail what
+happens in the two kinds of out of bounds scenarios that would raise an
+error -- `Failure` and `Invalid_argument`. You can pattern match against
+those by declaring the respective cases as exception instances and then
+returning the desired values in response.
+
+[source
+code](https://reasonml.github.io/en/try.html?reason=DYUwLgBAbghsCuIDOAuCwCWSwAoMDswBKCAXggG0BGAGgGYaBWGgdhoE4BdAbgChfQkAObgAanERkIOTNhoQCAExAAPEqQB8EAN68IEJAHcMYAMYALaQBksYAHSFzOWAmTylqoiV36APtAkQMi0XRD0If1VTEAAHMAwAe3wIADEYDAQAJxAcACJHXPUtAAZwyJVouMTkgEl8FwxFAH0YTKF4AFsQQjybbAcwc0LgiFL9AF8+Sf4AKSQ7YAShHBEwcVdnQKR5Yq8+OYWllbFAzddtiCo93gPF5dX1xDPEC8Zr26OH09C3CABaK5EbhAA)