Not all error messages are capitalized

[Presskopp]

Let's capitalize all of them.

I didn't touch class-pop3.php which has some more..; not sure if including ms-deprecated.php is needed here.

Related #42941

[SergeyBiryukov]

In 47154:

Text Changes: Capitalize error messages consistently.

Props Presskopp.
Fixes #42945.

[SergeyBiryukov]

In 47155:

Tests: Update the expected message in test_with_draft_post().

Follow-up to [47154].

See #42945.

[SergeyBiryukov]

In 47156:

Accessibility: Text Changes: Use sentence case for the word Error in various error messages, instead of all caps.

Using all caps should be avoided for better readability and because screen readers may pronounce all-caps words as abbreviations.

Props afercia, ryokuhi, sabernhardt, garrett-eclipse.
See #47656, #43037, #42945.

[afercia]

In 48059:

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.