Language Pack Upgrader can produce a fatal error

[ocean90]

Language_Pack_Upgrader::bulk_upgrade() returns a WP_Error object when we can't create a language dir.
Language_Pack_Upgrader::upgrade() is just a wrapper for Language_Pack_Upgrader::bulk_upgrade() and returns the first item from $result. That works if $result is an array or even false, but not if it's a WP_Error object:

Fatal error: Cannot use object of type WP_Error as array in /src/wp-admin/includes/class-wp-upgrader.php on line 1229

Introduced in [25633]/#22704.

[ocean90]

Moving to 4.0 because of #29397.

[nacin]

Oof, looks good.

[ocean90]

In 29675:

Language Pack Upgrader: Check return value before accessing the first array key.

Prevents a fatal error when Language_Pack_Upgrader::bulk_upgrade() returns a WP_Error object.

fixes #29470.

[jdgrimes]

#29089 was marked as a duplicate.