UploadBase: Return 'was-deleted' warning in addition to 'exists-normalized', not...

UploadBase: Return 'was-deleted' warning in addition to 'exists-normalized', not instead of

The 'was-deleted' warning was generated by getExistsWarning(), which
was returning immediately if this was found to be the case. A bunch of
later checks were incorrectly skipped, in particular 'exists-normalized',
which was resulting in UploadWizard incorrectly ignoring that problem.

I'm not sure why that was part of getExistsWarning() at all, it
doesn't seem very relevant. For that matter, neither do the 'thumb',
'thumb-name' and 'bad-prefix' warnings that it also generates, but
this should not be a problem in practice and so I'm leaving them alone.

Other than by allowing some more warning types to appear together or
in different order, this should not affect action=upload API output or
Special:Upload (which was updated appropriately). It does affect
'action=query&prop=imageinfo' output's 'html' property (used for AJAX
checks on Special:Upload), which no longer includes the 'was-deleted'
warning; this was never specified anywhere and just a side-effect.

Bug: T48741
Change-Id: I3686ee8ffd635f5f06f51971b6f16e3e66f33a9e