Include the property name in the error message when a single-valued property does not have exactly one value

Review at http://gwt-code-reviews.appspot.com/1348801

Review by: jat@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@9702 8db76d5a-ed1c-0410-87a9-c151d255dfc7
diff --git a/dev/core/src/com/google/gwt/core/ext/linker/impl/StandardConfigurationProperty.java b/dev/core/src/com/google/gwt/core/ext/linker/impl/StandardConfigurationProperty.java
index 2f1c3a2..2097b55 100644
--- a/dev/core/src/com/google/gwt/core/ext/linker/impl/StandardConfigurationProperty.java
+++ b/dev/core/src/com/google/gwt/core/ext/linker/impl/StandardConfigurationProperty.java
@@ -37,8 +37,8 @@
       throw new IllegalArgumentException("values is null");
     }
     if (!p.allowsMultipleValues() && values.size() != 1) {
-      throw new IllegalArgumentException(
-          "p is single-valued but values.size != 1");
+      throw new IllegalArgumentException(p.getName()
+          + " property requires exactly one value but values = " + values);
     }
   }