Handle sketch with only invalid code filenames

Previously, this would error out with an index out of bounds exception.
Now, an IOException is thrown, which is properly handled further up the
call chain.

app/src/processing/app/Sketch.java

@@ -151,7 +151,7 @@ public class Sketch {
    * Another exception is when an external editor is in use,
    * in which case the load happens each time "run" is hit.
    */
-  protected void load() {
+  protected void load() throws IOException {
     codeFolder = new File(folder, "code");
     dataFolder = new File(folder, "data");
 
@@ -193,6 +193,10 @@ public class Sketch {
         }
       }
     }
+
+    if (codeCount == 0)
+      throw new IOException(_("No valid code files found"));
+
     // Remove any code that wasn't proper
     code = (SketchCode[]) PApplet.subset(code, 0, codeCount);
 
@@ -1258,7 +1262,7 @@ public class Sketch {
    * When running from the editor, take care of preparations before running
    * the build.
    */
-  public void prepare() {
+  public void prepare() throws IOException {
     // make sure the user didn't hide the sketch folder
     ensureExistence();