Consolidate some strings related to user login

[pavelevap]

See attached patch. These corrected strings have two variants only with small changes. As a result of suggested unification, we can remove several similar strings.

[ocean90]

e-mail vs email: #14366

[SergeyBiryukov]

<strong>ERROR</strong>: The e-mail address isn&#8217;t correct.
<strong>ERROR</strong>: The email address isn&#8217;t correct.

One of these strings is in admin-$locale.po, the other one is in $locale.po. I guess they will remain in both files. However, there's a similar string in admin-$locale.po that can be merged:
​http://core.trac.wordpress.org/browser/tags/3.4.1/wp-admin/includes/ms.php#L225

Added in 21728.patch​. "e-mail" vs. "email" is currently inconsistent, but that can be left for #14366.

The requested user does not exist.
That user does not exist.

Please enter a correct email address.
Please enter a valid email address.

<strong>ERROR</strong>: This username is already registered, please choose another one.
<strong>ERROR</strong>: This username is already registered. Please choose another one.

Same as above, they're in different files. I guess it still might make sense to consolidate them for easier translation.

[pavelevap]

Thank you, Sergey! I did not notice this string in MS admin.

I think that merged strings will remain only in one file, see related ticket #20881.

Yes, the main purpose of this ticket was not problem "email" versus "e-mail", but only one small step to consolidate similar strings and make translators a little happier...

[pavelevap]

Patch updated, there are more strings to remove...

[ryan]

In [22124]:

Consolidate some strings. Props pavelevap, SergeyBiryukov. see #21728