Move exception text into exception object

Library/Homebrew/download_strategy.rb

@@ -184,12 +184,7 @@ class CurlDownloadStrategy < AbstractDownloadStrategy
           had_incomplete_download = false
           retry
         else
-          if @url =~ %r[^file://]
-            msg = "File does not exist: #{@url.sub(%r[^file://], "")}"
-          else
-            msg = "Download failed: #{@url}"
-          end
-          raise CurlDownloadStrategyError, msg
+          raise CurlDownloadStrategyError.new(@url)
         end
       end
       ignore_interrupts { temporary_path.rename(cached_location) }

Library/Homebrew/exceptions.rb

@@ -208,7 +208,16 @@ class DownloadError < RuntimeError
 end
 
 # raised in CurlDownloadStrategy.fetch
-class CurlDownloadStrategyError < RuntimeError; end
+class CurlDownloadStrategyError < RuntimeError
+  def initialize(url)
+    case url
+    when %r[^file://(.+)]
+      super "File does not exist: #{$1}"
+    else
+      super "Download failed: #{url}"
+    end
+  end
+end
 
 # raised by safe_system in utils.rb
 class ErrorDuringExecution < RuntimeError