Improve error message when 'del' keyword is used

--
MOS_MIGRATED_REVID=135784854

src/main/java/com/google/devtools/build/lib/syntax/Parser.java

diff --git a/src/main/java/com/google/devtools/build/lib/syntax/Parser.java b/src/main/java/com/google/devtools/build/lib/syntax/Parser.java
index 73da09b..18c2dbf 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/Parser.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/Parser.java
@@ -346,11 +346,14 @@
     String error;
     switch (token.kind) {
       case ASSERT: error = "'assert' not supported, use 'fail' instead"; break;
-      case TRY: error = "'try' not supported, all exceptions are fatal"; break;
+      case DEL:
+        error = "'del' not supported, use '.pop()' to delete an item from a dictionary or a list";
+        break;
       case IMPORT: error = "'import' not supported, use 'load' instead"; break;
       case IS: error = "'is' not supported, use '==' instead"; break;
       case LAMBDA: error = "'lambda' not supported, declare a function instead"; break;
       case RAISE: error = "'raise' not supported, use 'fail' instead"; break;
+      case TRY: error = "'try' not supported, all exceptions are fatal"; break;
       case WHILE: error = "'while' not supported, use 'for' instead"; break;
       default: error = "keyword '" + token.kind.getPrettyName() + "' not supported"; break;
     }