find-unneeded-includes: Make the output user friendlier

This way it's easy to copy-paste the problematic command
for further investigation of IWYUs proposals

Change-Id: I9e7403f0f05e64e562441941f00127a62bf15265
Reviewed-on: https://gerrit.libreoffice.org/54560Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>

bin/find-unneeded-includes

@@ -165,7 +165,7 @@ def run_tool(task_queue, failed_files):
             p = subprocess.Popen(invocation, shell=True, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
             retcode = processIWYUOutput(p.communicate()[0].decode('utf-8').splitlines(), moduleRules)
             if retcode != 0:
-                print("ERROR: '" + invocation + "' found unused includes.")
+                print("ERROR: The following command found unused includes:\n" + invocation)
                 failed_files.append(invocation)
         task_queue.task_done()