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.
```

2 files changed, 11 insertions, 3 deletions

diff --git a/activerecord/test/cases/connection_adapters/connection_handlers_multi_db_test.rb b/activerecord/test/cases/connection_adapters/connection_handlers_multi_db_test.rb
index 865aacc1b5..8988755d24 100644
--- a/activerecord/test/cases/connection_adapters/connection_handlers_multi_db_test.rb
+++ b/activerecord/test/cases/connection_adapters/connection_handlers_multi_db_test.rb
@@ -336,13 +336,13 @@ module ActiveRecord
       end
 
       def test_calling_connected_to_on_a_non_existent_handler_raises
-        error = assert_raises ArgumentError do
+        error = assert_raises ActiveRecord::ConnectionNotEstablished do
           ActiveRecord::Base.connected_to(role: :reading) do
-            yield
+            Person.first
           end
         end
 
-        assert_equal "The reading role does not exist. Add it by establishing a connection with `connects_to` or use an existing role (writing).", error.message
+        assert_equal "No connection pool with 'primary' found for the 'reading' role.", error.message
       end
     end
   end
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