Made secret adapters raise a meaningful error if the required CLI is not installed

lib/kamal/secrets/adapters/base.rb

@@ -2,6 +2,7 @@ class Kamal::Secrets::Adapters::Base
   delegate :optionize, to: Kamal::Utils
 
   def fetch(secrets, account:, from: nil)
+    check_dependencies!
     session = login(account)
     full_secrets = secrets.map { |secret| [ from, secret ].compact.join("/") }
     fetch_secrets(full_secrets, account: account, session: session)
@@ -15,4 +16,8 @@ class Kamal::Secrets::Adapters::Base
     def fetch_secrets(...)
       raise NotImplementedError
     end
+
+    def check_dependencies!
+      raise NotImplementedError
+    end
 end