[buildcore] Cleanup pybb error handling and type annotations

deps/buildcore/scripts/pybb.py

@@ -8,7 +8,7 @@
 #  file, You can obtain one at http://mozilla.org/MPL/2.0/.
 #
 
-# "Python Busy Box" - adds cross platform equivalents to Unix commands that
+# "Python Busy Box" - adds cross-platform equivalents to Unix commands that
 #                     don't translate well to that other operating system
 
 import os
@@ -17,7 +17,7 @@ import subprocess
 import sys
 
 
-def cat(paths: str) -> int:
+def cat(paths: [str]) -> int:
     for path in paths:
         try:
             with open(path) as f:
@@ -109,11 +109,15 @@ def main():
         err = cmake_build(sys.argv[2], sys.argv[3] if len(sys.argv) > 3 else None)
     elif sys.argv[1] == 'cat':
         err = cat(sys.argv[2:])
-    sys.exit(err)
+    else:
+        sys.stderr.write('Command not found\n')
+        err = 1
+    return err
 
 
 if __name__ == '__main__':
     try:
-        main()
+        err = main()
+        sys.exit(err)
     except KeyboardInterrupt:
         sys.exit(1)