This required changing the import script in two ways:
- we should use the `default` branch and not the `tip` tag since the
latter may refer to another branch than `default` (this is the case for
the `vi` locale, which caused in the files to be overwritten with
incorrect contents since `tip` referred to the
`THUNDERBIRD600b1_2018031614_RELBRANCH` branch);
- we should check if the response code is indeed 200 because recently a
script removed all empty localization files upstream (refer to
https://bugzilla.mozilla.org/show_bug.cgi?id=1443175).
