Refine error codes throughout the upgrader so we can better detect at what stage updates fail.

see #22704.

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git-svn-id: http://core.svn.wordpress.org/trunk@25676 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Silence possible warnings from is_dir(). fixes #25572.

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git-svn-id: http://core.svn.wordpress.org/trunk@25677 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Fourteen: remove header text customization options, fixes #25540. Props celloexpressions, obenland.
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git-svn-id: http://core.svn.wordpress.org/trunk@25678 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Fourteen: remove unneeded sidebar file. Props iamtakashi, see #25565.

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Update the basic browser detection to detect IE11. Fixes #25562.
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git-svn-id: http://core.svn.wordpress.org/trunk@25680 1a063a9b-81f0-0310-95a4-ce76da25c4cd