Hapistrano can fail at rollback if `ctoken` points to a release that does not exists

[CristhianMotoche]

Hapistrano uses ctokens to know which deployments finished successfully. After that, they're used to rollback to a successful release. However, we're not validating if the path that is generated for the rollback exists and ln doesn't fail if the target directory/file doesn't exists. Therefore, we should add a validation before that symlink is created. If the file doesn't exist, we should stop the rollback and notify the user.

This could happen if at some point the ctokens are manually manipulated or if a release is deleted and so are the ctokens.

Example:

/var/project/myproject
     /ctokens/
         20220102 # Just one ctoken pointing to an non-existing release
     /releases/
         20220103
         20220104
     current -> /var/project/myproject/releases/20220104

Executing hap rollback will cause:

/var/project/myproject
     /ctokens/
         20220102 # Just one ctoken pointing to an non-existing release
     /releases/
         20220103
         20220104
     current -> /var/project/myproject/releases/20220102  # This symlink doesn't exist but `ln` doesn't care and doesn't fail.

The current symlink points to a directory that doesn't exists. We could face errors later because of this.