Added MustacheContextException

Solving need to distinguish context errors from environment errors.
  The exception has fields for the offending key and line number. This
  is a first step towards internationalization of messages.

src/main/java/com/samskivert/mustache/Mustache.java

@@ -609,8 +609,8 @@ public class Mustache
         @Override public void execute (Template tmpl, Template.Context ctx, Writer out)  {
             Object value = tmpl.getValueOrDefault(ctx, _name, _line);
             if (value == null) {
-                throw new MustacheException(
-                    "No key, method or field with name '" + _name + "' on line " + _line);
+                throw new MustacheContextException(
+                    "No key, method or field with name '" + _name + "' on line " + _line, _name, _line);
             }
             String text = String.valueOf(value);
             write(out, _escapeHTML ? escapeHTML(text) : text);