b4-wrapper-poky.py: output errors to stderr

Print error messages to stderr instead of stdout.

The commands run as part of send-auto-cc-cmd and send-auto-to-cmd b4
commands will make b4 raise an Exception if they return a non-zero
return code and it will only print the content of stderr before that.

Because print defaults to stdout, b4 would raise the exception and not
tell the user why. This commit should now provide a hint to the user
about what went wrong.

Reported-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

Author: QSchulz

scripts/b4-wrapper-poky.py

@@ -53,7 +53,7 @@
 ref = None
 
 if not shutil.which("lsdiff"):
-    print("lsdiff missing from host, please install patchutils")
+    print("lsdiff missing from host, please install patchutils", file=sys.stderr)
     sys.exit(-1)
 
 try:
@@ -80,7 +80,7 @@
                 break
 
         if not ref:
-            print("Failed to find ref to cover letter (References:)...")
+            print("Failed to find ref to cover letter (References:)...", file=sys.stderr)
             sys.exit(-2)
 
         ref = ref.group(1)
@@ -143,7 +143,8 @@
 
     if cmd == "prep-perpatch-check-cmd":
         if len(projs) > 1:
-            print(f"Diff spans more than one project ({', '.join(sorted(projs))}), split into multiple commits...")
+            print(f"Diff spans more than one project ({', '.join(sorted(projs))}), split into multiple commits...",
+                  file=sys.stderr)
             sys.exit(-3)
 
         # No need to check other patches in the series as there aren't any
@@ -164,7 +165,8 @@
         series_check.write_text('\n'.join(uniq_series_projs))
 
         if len(uniq_series_projs) > 1:
-            print(f"Series spans more than one project ({', '.join(sorted(uniq_series_projs))}), split into multiple series...")
+            print(f"Series spans more than one project ({', '.join(sorted(uniq_series_projs))}), split into multiple series...",
+                  file=sys.stderr)
             sys.exit(-4)
     else:  # send-auto-cc-cmd / send-auto-to-cmd
         ml_projs = {