Dear mrutherford ,
One of the error handling structure can be as below,
At least folder
\_ All folder (for processing flow)
|_ All folder (for error handling)
The 2nd All folder will be executed if any assertion in 1st All folder fail. If 1st All folder is successful, the 2nd All folder will be skipped and continue the next block.
For your case, put route via http assertion under 1st All folder, and put send email alert assertion under 2nd All folder.
There is other way for error handling, such as using custom error response assertion.
Regards,
Mark