ui: Don't report "build failed:" for daemon error messages.

Until now we'd get things like:

  guix build: error: build failed: build of `/gnu/store/….drv' failed

or:

  $ guix gc -d /sdf
  guix gc: error: build failed: path `/sdf' is not in the store

which is kinda ridiculous.

* guix/ui.scm (call-with-error-handling): Remove "build failed:" prefix
for 'store-protocol-error?'.

guix/ui.scm

@@ -690,7 +690,7 @@ or remove one of them from the profile.")
                     (strerror (store-connection-error-code c))))
             ((store-protocol-error? c)
              ;; FIXME: Server-provided error messages aren't i18n'd.
-             (leave (G_ "build failed: ~a~%")
+             (leave (G_ "~a~%")
                     (store-protocol-error-message c)))
             ((derivation-missing-output-error? c)
              (leave (G_ "reference to invalid output '~a' of derivation '~a'~%")