mergeMessageFileList.php: abort on read error

If there is a parse error or if one of the files in the extension list
doesn't exist, exit with an error. mw-update-l10n has set -e so it will
abort without syncing the new file.

Change-Id: Idaad65783127b075626c102a8dc02e22df1588b7
Merge "mergeMessageFileList.php: abort on read error"