kernel: Handle and report build errors when creating an environment.

* guix-jupyter-kernel.scm (reply-execute-request): Add 'guard' clause
for 'store-protocol-error?'.

guix-jupyter-kernel.scm

@@ -485,6 +485,13 @@ stripped."
                                        " of package " (code ,package)
                                        " not found.")
                                  count)
+                    state))
+                 ((store-protocol-error? c)
+                  (let ((string (store-protocol-error-message c)))
+                    (reply-shtml kernel message
+                                 `(div (@ (class "ansi-red-fg"))
+                                       "Error: " ,string)
+                                 count)
                     state)))
          (let ((state (ensure-proxy-state-inferior state)))
            (match (lookup-proxied name state)