fix(copy): fix null pointer in copy method error checking

A nullpointer exception was being thrown in copySrcToDest function when a copy error was detected and its message was interpreted.

Author: hayuki

src/copy.ts

@@ -143,7 +143,7 @@ function copySrcToDest(context: BuildContext, src: string, dest: string, filter:
 
     fs.copy(src, dest, opts, (err) => {
       if (err) {
-        if (err.message.indexOf('ENOENT') > -1) {
+        if (err.message && err.message.indexOf('ENOENT') > -1) {
           resolve({ success: false, src: src, dest: dest, errorMessage: `Error copying "${src}" to "${dest}": File not found`});
         } else {
           resolve({ success: false, src: src, dest: dest, errorMessage: `Error copying "${src}" to "${dest}"`});