Update text on `dotnet tool install` error message

### When a tool is already installed, the error message could be even better

I received a question via email regarding the error message below that appears when a tool is already installed. It would be very easy for the message to also point folks in the right direction.

It's easy to forget that not everyone who uses .NET is familiar with using CLIs, as many things are happily just "follow the recipe" (yeah!!). So, I think this is worth doing. I think the local fix is worth doing even if the broader fix is also included.

### Describe the solution you'd like:

Current text:

"Tool `` is already installed"

Proposed text:

"Tool `` is already installed. Use 'dotnet tool update' if you would like to update to a newer version."

ALSO:

Consider adding a message similar to the following to all failed CLI commands:

"For more information, use 'dotnet --help' or

No file or test is named. Search the SDK repository for the existing dotnet tool install error text, then inspect the surrounding command and its tests; done means the already-installed message includes the proposed update guidance, while the broader failed-command help text remains a separate consideration.

Written by the indexing model from the issue text.