libdpkg: Always prefix "error" to fatal error message output

debian/changelog

@@ -15,6 +15,7 @@ dpkg (1.16.0) UNRELEASED; urgency=low
   * Add missing options to update-alternative --help output.
   * Count “conffile name is duplicated” for dpkg-deb warning count summary.
   * Improve and clarify strings for translation. Closes: #604914
+  * Prefix all fatal error messages with “error: ”.
 
   [ Raphaël Hertzog ]
   * Fail properly when debian/source/format is empty. Closes: #600854

lib/dpkg/ehandle.c

@@ -351,7 +351,7 @@ catch_fatal_error(void)
 void
 print_fatal_error(const char *emsg, const char *contextstring)
 {
-  fprintf(stderr, "%s: %s\n",thisname,emsg);
+  fprintf(stderr, _("%s: error: %s\n"), thisname, emsg);
 }
 
 void