Clarified an error messages and changed Rubocop parameter.

A more meaningful error message than "Cannot calculate the Docker tag for <var>"
Sent unnecessary output in ensure_can_access_dockerd to /dev/null
Increased permitted MethodLength on Rubocop to 15

Fixes #16

Signed-off-by: ZeTopHat <chamilton@suse.com>

.rubocop.yml

@@ -3,3 +3,5 @@ AllCops:
     - Vagrantfile
     - sle2docker.gemspec
     - lib/templates/**/*
+Metrics/MethodLength:
+  Max: 15

lib/sle2docker/cli.rb

@@ -54,7 +54,7 @@ def version
     private
 
     def ensure_can_access_dockerd
-      output = `docker info`
+      output = `docker info 2>/dev/null`
       raise output if $CHILD_STATUS.exitstatus.nonzero?
     end
 

lib/sle2docker/prebuilt_image.rb

@@ -110,7 +110,8 @@ def compute_repository_and_tag
       match = regexp.match(@image_name)
       match.nil? &&
         raise(DockerTagError,
-              "Cannot calculate the Docker tag for #{@image_name}")
+              "Docker image #{@image_name} not found. \
+               Run sle2docker list to check which docker images are available.")
 
       @repository = "suse/#{match['name']}"
       @tag        = match['version']