I18N: Remove the "Error:" prefix from error messages.

For a number of years, most of the WordPress error messages have been prefixed with "Error:". However, these messages appear in a context where it's already clear an error occurred. Whether it's an error, a warning, or any other classification, that's not so relevant for users. The content of the message is the relevant part. The "Error:" prefix doesn't add great value while it does add unnecessary complexity for the message readability.

Also, revises some of these messages to improve clarity and removes HTML from translatable strings.

Props garrett-eclipse, ramiy, SergeyBiryukov, afercia, sabernhardt, quadthemes, audrasjb. 
See #47003, #43037, #42945, #15887.
Fixes #47656.

Built from https://develop.svn.wordpress.org/trunk@48059


git-svn-id: https://core.svn.wordpress.org/trunk@47826 1a063a9b-81f0-0310-95a4-ce76da25c4cd