Add upfront validations in deploy action

[neel-astro]

Description:

There is a scope for adding validation around user input and validation on the results of some of the checks we do upfront instead of waiting for the workflow to fail unexpectedly. Some of these scenarios to cover are:

• add validation on root-folder, can't have it start with "/", and also can't set it to a non-existent folder
• astro deployment inspect commands do not return with exit code 1 if the key does not exist, i.e. in case of deployment is not found in the workspace, so where we are assigning variable values, needs to handle for failure scenarios separately, or we could also update the astro deployment inspect logic to return with exit code 1.