Fix printing MultipleVersionsInstalledError details

The refactor in 6e8f5d0958247e4b4d629866099ed2836a0e0863 means that the
exception no longer exposes the name of the package with multiple versions,
and as a result the rescuer is unable to print this information.

Because we now have a path in which MultipleVersionsInstalledError doesn't
have the name at all, we can't reasonably restore the old behaviour.
And since rack resolution happens purely internal to the function that
raises the exception, the caller has no way to know what name to use.
However, since the exception text gets printed anyway, we can just move
this text into the exception itself.

Fixes the following error:

```
Error: mpd has multiple installed versions
Error: undefined method `name' for #<MultipleVersionsInstalledError:0x00007fc6009d8870>
/usr/local/Homebrew/Library/Homebrew/cmd/uninstall.rb:137:in `rescue in uninstall'
/usr/local/Homebrew/Library/Homebrew/cmd/uninstall.rb:135:in `uninstall'
/usr/local/Homebrew/Library/Homebrew/brew.rb:119:in `<main>'
```

Library/Homebrew/cli/named_args.rb

@@ -164,7 +164,10 @@ module Homebrew
             end
 
             unless (prefix = f.latest_installed_prefix).directory?
-              raise MultipleVersionsInstalledError, "#{rack.basename} has multiple installed versions"
+              raise MultipleVersionsInstalledError, <<~EOS
+                #{rack.basename} has multiple installed versions
+                Run `brew uninstall --force #{rack.basename}` to remove all versions.
+              EOS
             end
 
             Keg.new(prefix)

Library/Homebrew/cmd/uninstall.rb

@@ -134,7 +134,6 @@ module Homebrew
     )
   rescue MultipleVersionsInstalledError => e
     ofail e
-    puts "Run `brew uninstall --force #{e.name}` to remove all versions."
   ensure
     # If we delete Cellar/newname, then Cellar/oldname symlink
     # can become broken and we have to remove it.

Library/Homebrew/test/exceptions_spec.rb

@@ -3,9 +3,19 @@
 require "exceptions"
 
 describe MultipleVersionsInstalledError do
-  subject { described_class.new("foo has multiple installed versions") }
+  subject {
+    described_class.new <<~EOS
+      foo has multiple installed versions
+      Run `brew uninstall --force foo` to remove all versions.
+    EOS
+  }
 
-  its(:to_s) { is_expected.to eq("foo has multiple installed versions") }
+  its(:to_s) {
+    is_expected.to eq <<~EOS
+      foo has multiple installed versions
+      Run `brew uninstall --force foo` to remove all versions.
+    EOS
+  }
 end
 
 describe NoSuchKegError do