improve error message

cmdline/apt-cdrom.cc

@@ -67,7 +67,8 @@ void pkgCdromTextStatus::Prompt(const char *Text)
    char C;
    cout << Text << ' ' << flush;
    if (read(STDIN_FILENO,&C,1) < 0)
-      _error->Errno("pkgCdromTextStatus::Prompt", "failed to prompt");
+      _error->Errno("pkgCdromTextStatus::Prompt", 
+                    "Failed to read from standard input (not a terminal?)");
    if (C != '\n')
       cout << endl;
 }