Add ConnectionRefused exception

* required to fix retry+connection initialization corner case.

src/db/error.cr

@@ -11,6 +11,9 @@ module DB
   class PoolRetryAttemptsExceeded < Error
   end
 
+  # Raised when an established connection is lost
+  # probably due to socket/network issues.
+  # It is used by the connection pool retry logic.
   class ConnectionLost < Error
     getter connection : Connection
 
@@ -18,6 +21,12 @@ module DB
     end
   end
 
+  # Raised when a connection is unable to be established
+  # probably due to socket/network or configuration issues.
+  # It is used by the connection pool retry logic.
+  class ConnectionRefused < Error
+  end
+
   class Rollback < Exception
   end
 end

src/db/pool.cr

@@ -93,6 +93,10 @@ module DB
           # and remove it from the known pool.
           delete(e.connection)
           e.connection.close
+        rescue e : ConnectionRefused
+          # a ConnectionRefused means a new connection
+          # was intented to be created
+          # nothing to due but to retry soon
         end
       end
       raise PoolRetryAttemptsExceeded.new