[various] Improve documentation

Output file must be preallocated.

features/step_definitions/thin_steps.rb

@@ -24,6 +24,11 @@ Given(/^a corrupt superblock$/) do
   end
 end
 
+Given(/^a tiny file$/) do
+  run_simple("rm -f tiny")
+  run_simple("fallocate -l 123 tiny")
+end
+
 When(/^I run thin_check with (.*?)$/) do |opts|
   run_simple("thin_check #{opts} #{dev_file}", false)
 end

features/thin_restore.feature

@@ -55,6 +55,18 @@ Feature: thin_restore
     No output file provided.
     """
 
+  Scenario: tiny output file
+    Given a tiny file
+    When I run thin_restore with -i metadata.xml -o tiny
+    Then it should fail with:
+    """
+    Output file too small.
+
+    The output file should either be a block device,
+    or an existing file.  The file needs to be large
+    enough to hold the metadata.
+    """
+
   Scenario: --quiet is accepted
     Given valid thin metadata
     When I run thin_restore with -i metadata.xml -o metadata.bin --quiet