Improve report error about missing deployment handler

[cdeneux]

I try to deploy my roboconf application on Docker containers.

If the target docker is not installed in the Roboconf DM, I'm not able to deploy my VMs without explanations from the Roboconf web-console. The error about the missing deployment target is only logged in Roboconf log files.

Please add an error displayed by the Roboconf web-console about the missing deployment target when loading the application template.

[vincent-zurczak]

Planned for the next version.
See roboconf/roboconf-platform#814