Add a meaningful message when IaaS capabilities are reached

[vincent-zurczak]

When a VM fails to be created, Roboconf marks the VM in error but does not explain why. We might add a better error message for users.

[vincent-zurczak]

• We should create new error codes.
• Change the constructor for TargetException and set an error code.
• Propagate the error code to the instance's data map.
• Interpret the error code in the web admin and add a tool tip or something to display it.

No that much complex in fact. 😉