Clean up script entry points

Two factors here:
1. Ensure that each script has an exlicit main() function
2. Ensure that the return value of main() is the return value of the
   program when the script is invoked directly.

scripts/rosdistro

@@ -83,4 +83,4 @@ def main():
 
 
 if __name__ == "__main__":
-    main()
+    sys.exit(main())

scripts/rosdistro_build_cache

@@ -84,7 +84,7 @@ def main():
             ignore_local=args.ignore_local, include_source=args.include_source, debug=args.debug)
     except RuntimeError as e:
         print(str(e), file=sys.stderr)
-        sys.exit(1)
+        return 1
 
     for dist_name, cache in caches.items():
         if args.dist_names and dist_name not in args.dist_names:
@@ -101,4 +101,4 @@ def main():
 
 
 if __name__ == '__main__':
-    main()
+    sys.exit(main())

scripts/rosdistro_convert

@@ -257,10 +257,14 @@ def convert_doc(dist_name):
         f.write(yaml_str)
 
 
-if __name__ == '__main__':
+def main():
     targets = get_targets()
     for distro in ['groovy', 'hydro']:
         print('\nConverting "%s"\n' % distro)
         convert_release(distro, targets)
         convert_source(distro)
         convert_doc(distro)
+
+
+if __name__ == '__main__':
+    sys.exit(main())

scripts/rosdistro_freeze_source

@@ -95,4 +95,4 @@ def main():
 
 
 if __name__ == '__main__':
-    main()
+    sys.exit(main())

scripts/rosdistro_generate_cache

@@ -34,4 +34,4 @@ def main():
 
 
 if __name__ == "__main__":
-    main()
+    sys.exit(main())

scripts/rosdistro_migrate_to_rep_141

@@ -3,6 +3,7 @@
 import argparse
 import gzip
 import os
+import sys
 
 try:
     from cStringIO import StringIO
@@ -163,9 +164,13 @@ def get_dict_parts(d, keys):
     return data
 
 
-if __name__ == "__main__":
+def main():
     parser = argparse.ArgumentParser(description='Migrate the distros from REP 137 to REP 141.')
     parser.add_argument('index', help='The index.yaml file to migrate')
     args = parser.parse_args()
 
     migrate(args.index)
+
+
+if __name__ == "__main__":
+    sys.exit(main())

scripts/rosdistro_migrate_to_rep_143

@@ -1,6 +1,7 @@
 #!/usr/bin/env python3
 
 import argparse
+import sys
 
 from rosdistro.verify import _yaml_header_lines
 
@@ -32,9 +33,13 @@ def index_to_yaml(data):
     return yaml_str
 
 
-if __name__ == "__main__":
+def main():
     parser = argparse.ArgumentParser(description='Migrate the distros from REP 141 to REP 143.')
     parser.add_argument('index', help='The index.yaml file to migrate')
     args = parser.parse_args()
 
     migrate(args.index)
+
+
+if __name__ == "__main__":
+    sys.exit(main())

scripts/rosdistro_reformat

@@ -55,7 +55,7 @@ def parse_args(args=sys.argv[1:]):
     return parser.parse_args(args)
 
 
-if __name__ == '__main__':
+def main():
     args = parse_args()
     index = args.index
     if os.path.isfile(index):
@@ -64,4 +64,8 @@ if __name__ == '__main__':
         success = verify_files_identical(index)
     else:
         success = reformat_files(index)
-    sys.exit(0 if success else 1)
+    return 0 if success else 1
+
+
+if __name__ == '__main__':
+    sys.exit(main())