Devc uses previous Docker image if updated Dockerfile has an error during rebuild

[ericglau]

1. Start dev mode with containers, using a valid Dockerfile
2. Change the Dockerfile to have some invalid commands or Dockerfile syntax errors
3. Dev mode tries to rebuild the Dockerfile, shows an error, but then starts the previous container image

This is misleading since the user might not see the docker build failure, and wonder why their changes are not being reflected in the server/container that is being run.

[ericglau]

Ideally dev mode should not exit in this case, but do not run docker run and still watch for changes in the Dockerfile and do further rebuilds. It should not start the previous image if the Docker build failed though.