Explain the connection pool error message better

The previous message was misleading (especially for Ops guys) when
diagnosing problems related to the database connection.

The message was suggesting that the connection cannot be obtained which
normally assumes the need to look at the database.

But this isn't the case as the connection could not be retrieved from
the application's internal connection pool.

The new message should make it more explicit and remove the confusion.

activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb

@@ -197,7 +197,7 @@ def wait_poll(timeout)
 
             elapsed = Time.now - t0
             if elapsed >= timeout
-              msg = 'could not obtain a database connection within %0.3f seconds (waited %0.3f seconds)' %
+              msg = 'could not obtain a connection from the pool within %0.3f seconds (waited %0.3f seconds); all pooled connections were in use' %
                 [timeout, elapsed]
               raise ConnectionTimeoutError, msg
             end