createRelease.sh: added error checking in modify_files()

Otherwise, when inserting a multiline changelog sed would with this message:
  sed: -e expression #1, char 27: unterminated `s' command

And the script would continue with an unmodified CHANGELOG.md
For simmetry, added the same check to package.json, too

bin/createRelease.sh

@@ -66,8 +66,11 @@ function check_api_token {
 function modify_files {
   # Add changelog text to first line of CHANGELOG.md
   sed -i "1s/^/${changelogText}\n/" CHANGELOG.md
+  [[ $? != 0 ]] && echo "Aborting: Error modifying CHANGELOG.md" && exit 1
+
   # Replace version number of etherpad in package.json
   sed -i -r "s/(\"version\"[ ]*: \").*(\")/\1$VERSION\2/" src/package.json
+  [[ $? != 0 ]] && echo "Aborting: Error modifying package.json" && exit 1
 }
 
 function create_release_branch {