build-release-notes: fail if the directory does not exist

This was a combination of two problems: the python didn't throw an error
because apparently glob on a nonexistent directory doesn't crash, and
secondarily, bash ignores bad exit codes without `set -e` if they are
not in the final/only command.

Change-Id: I812bde7a4daee5c77ffe9d7c73a25fd14969f548

doc/manual/src/release-notes/meson.build

@@ -1,6 +1,8 @@
 rl_next_generated = custom_target(
   command : [
     'bash',
+    '-euo',
+    'pipefail',
     '-c',
     '''
       if type -p build-release-notes > /dev/null; then

maintainers/build-release-notes.py

@@ -40,7 +40,10 @@ def plural_list(strs: list[str]) -> str:
         return '{}{} and {}'.format(', '.join(strs[:-1]), comma, strs[-1])
 
 def run_on_dir(d):
-    paths = pathlib.Path(d).glob('*.md')
+    d = pathlib.Path(d)
+    if not d.is_dir():
+        raise ValueError(f'provided path {d} is not a directory')
+    paths = d.glob('*.md')
     entries = []
     for p in paths:
         try: