exceptions.rb: Add DownloadError

`DownloadError` is an exception that download stratigies can throw to indicate
that a fetch was incomplete due to a failure in communication.

The `curl` method in `utils.rb` has been upgraded to throw a `DownloadError` if
something bad happens to `curl` execution.

Library/Homebrew/exceptions.rb

@@ -112,3 +112,17 @@ class BuildError < Homebrew::InstallationError
     @command == './configure'
   end
 end
+
+class DownloadError < RuntimeError
+  attr :command
+  attr :args
+  attr :exit_status
+
+  def initialize cmd, args, status
+    @command = cmd
+    @args = args
+    args.map!{ |arg| arg.to_s.gsub " ", "\\ " }
+    super "#{cmd} #{args.join ' '}\nDownloader failed with exit status #{status}"
+    @exit_status = status
+  end
+end

Library/Homebrew/utils.rb

 require 'pathname'
+require 'exceptions'
 
 class Tty
   class <<self
@@ -104,10 +105,14 @@ def quiet_system cmd, *args
 end
 
 def curl *args
+  curl = Pathname.new '/usr/bin/curl'
+  raise "#{curl} is not an executable!" unless curl.exist? and curl.executable?
+
+  args = [HOMEBREW_CURL_ARGS, HOMEBREW_USER_AGENT, *args]
   # See https://github.com/mxcl/homebrew/issues/6103
   args << "--insecure" if MacOS.version < 10.6
 
-  safe_system '/usr/bin/curl', HOMEBREW_CURL_ARGS, HOMEBREW_USER_AGENT, *args unless args.empty?
+  raise DownloadError.new curl, args, $? unless Homebrew.system curl, *args
 end
 
 def puts_columns items, star_items=[]