update.authorize.inc log messages should be improved

Problem/Motivation

This issue was identified by catch in #2577407-194: Action of uploading module/theme files should consistently be called "Add", not "Install" → (item 2 in comment 194). In comment 195, jhodgdon suggested not fixing it in that issue because it would require a logic change and the patch was otherwise pure UI text updates. In comment 201, catch committed the patch for that issue and suggested a follow-up, so here it is!

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When you use the Update Manager to either download a new module/theme from drupal.org, or download updated files for an existing module/theme/profile, you get a message like this in the "log" section of the after-operations page (as of #2577407: Action of uploading module/theme files should consistently be called "Add", not "Install" → the wording was changed slightly; this is the current wording):

"Error adding / updating" [if there was a problem]
"Added / updated %project_name successfully" [if it worked]

These messages are in the file core/modules/update/update.authorize.inc. The same message is used whether you are adding a new module/theme or updating the files for a module/theme.

Elsewhere in that file and on other UI screens for the Update Manager module, we know whether we are adding or updating, and the messages either use the verb "add" or "update", and not a "add/update" as in this message. This message looks really low level, but is being presented in the UI, giving the impression of a thrown-together system built by nerdy software developers rather than a polished UI.

Steps to reproduce

Use the Update Manager page to either add a new module/theme (there are links from the Extend, Appearance, and Available Updates pages), or update the files for a module/theme (link on the Available Update page).

You'll see the message in the detailed log section of the results page.

Proposed resolution

It would be better if we fixed the logic here to use different messages for adding a new module/theme vs. updating an existing module/theme. The messages when you downloaded a new module/theme should use the verb "add", and when you updated should use the verb "update".

Remaining tasks

Make a patch.

User interface changes

More polished and professional looking UI messages.

API changes

TBD

Data model changes

None.

Release notes snippet

Probably not needed.