Cache SD API verification

[RunDevelopment]

Verify that an SD API service is available is now done manually via the verify_api_connection. Nodes that wish to only appear when an SD API is up should call this function before registering. This also allows us to define SD nodes that should appear even if no external API is up.

The main motivation for this function are logs, though. The error message for when no API service is available is very detailed. This is good, but the message is also repeated 4 times, which makes it spam the log output.

> Attempting to spawn backend...
> Backend: [6868] [ERROR] A critical error occurred when importing module nodes.nodes.external_stable_diffusion.img2img: 
If you want to use external stable diffusion nodes, run the Automatic1111 web ui with the --api flag, like so:

./webui.sh --api

ChaiNNer is currently configured to look for the API at http://127.0.0.1:7860.  If you
have it running somewhere else, you can change this using the STABLE_DIFFUSION_HOST and STABLE_DIFFUSION_PORT
environment variables.

18:06:40.038 > Backend: [6868] [ERROR] A critical error occurred when importing module nodes.nodes.external_stable_diffusion.interrogate: 
If you want to use external stable diffusion nodes, run the Automatic1111 web ui with the --api flag, like so:

./webui.sh --api

ChaiNNer is currently configured to look for the API at http://127.0.0.1:7860.  If you
have it running somewhere else, you can change this using the STABLE_DIFFUSION_HOST and STABLE_DIFFUSION_PORT
environment variables.

18:06:42.055 > Backend: [6868] [ERROR] A critical error occurred when importing module nodes.nodes.external_stable_diffusion.outpainting: 
If you want to use external stable diffusion nodes, run the Automatic1111 web ui with the --api flag, like so:

./webui.sh --api

ChaiNNer is currently configured to look for the API at http://127.0.0.1:7860.  If you
have it running somewhere else, you can change this using the STABLE_DIFFUSION_HOST and STABLE_DIFFUSION_PORT
environment variables.

18:06:44.127 > Backend: [6868] [ERROR] A critical error occurred when importing module nodes.nodes.external_stable_diffusion.txt2img: 
If you want to use external stable diffusion nodes, run the Automatic1111 web ui with the --api flag, like so:

./webui.sh --api

ChaiNNer is currently configured to look for the API at http://127.0.0.1:7860.  If you
have it running somewhere else, you can change this using the STABLE_DIFFUSION_HOST and STABLE_DIFFUSION_PORT
environment variables.

The new function will only use an error with the detailed error message the first time around.

@adodge Thoughts?

[adodge]

Nice! A good idea, well implemented.