Document error handling

Closes stephencelis#700

Author: jberkel

Documentation/Index.md

@@ -650,6 +650,20 @@ follow similar patterns.
 > // INSERT INTO "timestamps" DEFAULT VALUES
 > ```
 
+### Handling specific SQLite errors
+
+You can pattern match on the error to selectively catch SQLite errors:
+
+```swift
+do {
+    try db.run(users.insert(email <- "[email protected]"))
+    try db.run(users.insert(email <- "[email protected]"))
+} catch let Result.error(_, code, _) where code == SQLITE_CONSTRAINT {
+    print("constraint failed")
+} catch let error {
+    print("insertion failed: \(error)")
+}
+```
 
 ### Setters
 

Tests/SQLiteTests/QueryTests.swift

@@ -518,4 +518,16 @@ class QueryIntegrationTests : SQLiteTestCase {
             }
         }
     }
+
+    func test_catchConstraintError() {
+        try! db.run(users.insert(email <- "[email protected]"))
+        do {
+            try db.run(users.insert(email <- "[email protected]"))
+            XCTFail("expected error")
+        } catch let Result.error(_, code, _) where code == SQLITE_CONSTRAINT {
+            // expected
+        } catch let error {
+            XCTFail("unexpected error: \(error)")
+        }
+    }
 }