WP_Upgrader does not consistently call Skin methods

[rmccue]

In WP_Upgrader::run() (used by the installer/upgrader routines), the header/footer/before/after methods are not called consistently.

(header/before/error/after/footer are all references to the methods as part of the Skin object, stored in $this->skin)

• An error in the filesystem will cause a call to error without first calling header/before and without calling after/footer.
• An error in downloading or unpacking will cause calls to header/before before error, but only after is called after error.
• An error in actually "installing" (moving into place) or a successful install will cause calls to header/before (then error if errored), then after/footer

In order to actually make these possible to use consistently, the following order should always be followed:

1. header (if not doing a multiupgrade)
2. before
3. error (or other feedback calls; the meat of the processing)
4. after
5. footer (if not doing a multiupgrade)

[SergeyBiryukov]

First pass: 25254.diff​ (untested).

[dd32]

Looks good to me

[rmccue]

Looks good, but footer should only be called for non-multi (since that's the difference between after and footer)

[SergeyBiryukov]

Right, see 25254.2.diff​.

[SergeyBiryukov]

In 25370:

Consistently call skin methods in WP_Upgrader. fixes #25254.