#3613 fixing off-by-one error on compiler errors (#3692)

As both the python filename array and the Input_X indexing is 0 based, the `-1` operation is not needed.

fromFileArray in closure-compiler uses 0 based indexing. see:

https://github.com/google/closure-compiler/blob/09cca3b536923dfb86d4e2ea34c8ee97d3ab471c/src/com/google/javascript/jscomp/gwt/client/JsRunnerMain.java#L816

build.py

@@ -309,7 +309,7 @@ goog.provide('Blockly.utils.string');
     def file_lookup(name):
       if not name.startswith("Input_"):
         return "???"
-      n = int(name[6:]) - 1
+      n = int(name[6:])
       return filenames[n]
 
     if "serverErrors" in json_data: