Add exception handling for invalid key interpretation in PDT.

Assets/Cryville/Common/Pdt/PdtInterpreter.cs

@@ -233,6 +233,8 @@ namespace Cryville.Common.Pdt {
 						break;
 					case '}':
 						return result;
+					default:
+						throw new InvalidOperationException("Internal error: Invalid key interpretation");
 				}
 			}
 		}