Improve validation and error handling on install.php

[DrewAPicture]

As @nacin put in in 36:ticket:24078, install.php needs better ​poka-yoke — a Japanese term for 'mistake-proofing' — for error handling and validation, especially since 'admin' was removed as the default username in [24998].

Currently, displayed errors take the form of 'ERROR some error' which isn't great. That formatting needs improvement but it also seems like this might be a good opportunity introduce some in-line validation. Whether that be js with no-js fallbacks or what-have-you.

[dd32]

See also #15729 for DB credential validation

[DrewAPicture]

24982.diff​ is a compromise of sorts.

• It improves the wording of the errors so we can drop the 'ERROR' bit off the front
• It also marks the Username and Email fields with an asterisk, e.g. required
• And shuffles the fields so the two required Username and Email fields are grouped together

I think we should look at giving the install screen (and perhaps install.php itself) a facelift as part of the admin overhaul in 3.8. These small changes will be enough I think until we're ready to take more invasive steps to improve the experience.

[ocean90]

It also marks the Username and Email fields with an asterisk, e.g. required

For new users, that's not clear enough, since there is no explanation what the asterisk stands for. See comment_form() where we have sprintf( ' ' . __('Required fields are marked %s'), '<span class="required">*</span>' );.
Maybe we should also add aria-required="true".

The colors should match the one from div.error, so background-color: #ffebe8; and border-color: #c00;, and also lowercase, see ​codex.

Otherwise I'm fine with removing the ERROR prefix.

[nacin]

Good with 24982.3.diff​. Thank you for making .message pretty. :-)

[helen]

In 25620:

Better error messaging on install.php. props DrewAPicture, ocean90. fixes #24982.

[nacin]

In 25657:

End sentences in periods. see #24982.