Begin drafting try/catch proposal

proposal/try-catch.md

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+# Support `@try` and `@catch` flow controls for error handling: Draft 1
+
+*[(Issue)](https://github.com/sass/sass/issues/2619)*
+
+## Table of Contents
+
+## Background
+
+> This section is non-normative.
+
+In many situations, particularly when writing tests, it's useful to catch error
+and warning messages without breaking compilation. By adding `@try` and `@catch`
+at-rules similar to other control-flow syntax, authors will have more control
+over error handling, and the ability to test for error-cases.
+
+Many large Sass projects currently use wrapper functions and mixins to catch and
+manage errors, with a parameter to toggle between throwing or returning the
+error message. But that relies on the special error handling to be built into
+every individual function or mixin that might error. By adding global controls,
+error-handling can happen either where the error is generated, or where it is
+defined.
+
+## Summary
+
+> This section is non-normative.
+
+This proposal defines a pair of new at-rule directives that must be used
+together as a form of control. The `@try { ... } @catch { ... }` syntax captures
+any thrown output (error or warning messages) from inside the `@try` block, and
+allows the parser to continue compilation. The `@catch` block executes whenever
+an error or warning has been captured, allowing authors to determine how the
+error should be processed.
+
+```scss
+@try {
+  $test: my.function('bad arg');
+} @catch $error {
+  @include true.assert-equal(
+    $error,
+    "'bad arg' is not a valid argument for 'function'"
+  );
+}
+```
+
+## Syntax
+
+<x><pre>
+**TryCatchRule**    ::= '@try' '{'
+&#32;                     TryStatements
+&#32;                   '}' '@catch' exceptionVar? '{'
+&#32;                     CatchStatements
+&#32;                   '}'
+**exceptionVar**    ::= '$' Identifier
+</pre></x>
+
+## Semantics
+
+To execute a `TryCatchRule` `rule`:
+
+* Let `exceptions`
+
+* Execute the contents `try` of `rule`'s `TryStatements`
+
+  * If an `@error` or `@warn` rule `throw` is encountered:
+
+    * Let `exception` be a map with the same identifier as `exceptionVar`,
+      a 'name' `throw`s at-rule
+      
+      > Currently either `@error` or `@warn`
+
+    * Let `message` be the value of the `throw`'s `expression`
+
+    * If `name` is `@error`, ignore any remaining statements in `try`
+
+    * Otherwise complete compilation of the `TryStatements`
+
+    * If `exceptionVar` is defined
+      
+      * Let `exception` be a map variable with the same name as `exceptionVar`'s
+        identifier.
+
+      * Let `exception` have a key 'name' with the value of `name`
+
+      * Let `exception` have a key 'messsage' with the value of `message`
+
+      > assign `message` to identifier `catchMessage`...
+
+    > Execute the contents of `rule`'s `CatchStatements` with `catchMessage` and
+    >  `catchType`...
+
+  * Otherwise, ignore the remainder of the `TryCatchRule`
+