Fix error raised when handler doesn't exist

While working on another feature for multiple databases (auto-switching)
I observed that in development the first request won't autoload the
application record connection for the primary database and may not yet
know about the replica connection.

In my test application this caused the application to thrown an error if
I tried to send the first request to the replica before the replica was
connected. This wouldn't be an issue in production because the
application is preloaded.

In order to fix this I decided to leave the original error message and
delete the new error message. I updated the original error message to
include the `role` to make it a bit clearer that the connection isn't
established for that particular role.

The error now reads:

```
No connection pool with 'primary' found for the 'reading' role.
```

A single database application will continue uisng the original error
message:

```
No connection pool with 'primary' found.
```

1 files changed, 8 insertions, 0 deletions

diff --git a/activerecord/test/cases/unconnected_test.rb b/activerecord/test/cases/unconnected_test.rb
index 9eefc32745..f0a0e7f805 100644
--- a/activerecord/test/cases/unconnected_test.rb
+++ b/activerecord/test/cases/unconnected_test.rb
@@ -29,6 +29,14 @@ class TestUnconnectedAdapter < ActiveRecord::TestCase
     end
   end
 
+  def test_error_message_when_connection_not_established
+    error = assert_raise(ActiveRecord::ConnectionNotEstablished) do
+      TestRecord.find(1)
+    end
+
+    assert_equal "No connection pool with 'primary' found.", error.message
+  end
+
   def test_underlying_adapter_no_longer_active
     assert_not @underlying.active?, "Removed adapter should no longer be active"
   end