Plugins: Improve plugin install error message for non-site-owner admins

Expected behavior

As an admin who isn't the site owner on a WordPress.com Business site, I shouldn't be able to install the first plugin on the site. From the Adding plugins support doc:

Note: For a site with Business plan and multiple administrators on them, only the account that owns the site, purchases Business plan on it, and registers or maps a domain name to it, is able to install the first plugin or third-party theme. After the first plugin is installed, other administrators on the site will be able to install plugins on it as well.

When I try to install a plugin, I expect a clear message explaining that. Here's what Calypso shows when another admin tries to install a plugin:

Actual behavior

In the app, the error message is generic and confusing when I'm an admin on the site:

Plugin feature requires admin privileges.

Steps to reproduce the behavior

1. Create a WordPress.com site with the Business plan.
2. Invite another user to be an admin on the site.
3. Log in to the app as the second admin.
4. Go to My Sites > Plugins.
5. Select a plugin and try to install it.

Tested on iPhone XS, iOS 12.1, WPiOS 11.3.0.20181119

Reproduce the issue from My Sites > Plugins using a non-owner administrator on a WordPress.com Business site, then trace the app's generic "Plugin feature requires admin privileges" error. Update the install failure wording to explain the site-owner restriction for the first plugin, and verify the message during the reported installation flow.