raise explicit exception if EOF found when looking for changelog sections

Author: nickfyson

.github/update-release-branch.py

@@ -188,6 +188,8 @@ def process_changelog_for_backports(source_branch_major_version, target_branch_m
     # until we find the first section, just duplicate all lines
     while True:
       line = f.readline()
+      if not line:
+        raise Exception('Could not find the first changed section in CHANGELOG') # EOF
 
       if line.startswith('## '):
         line = line.replace(f'## {source_branch_major_version}', f'## {target_branch_major_version}')