ldd: slightly improve error message.

Before:

  $ ldd toybox
  toybox: not an ELF file

After

  $ ldd toybox
  toybox: cannot open

I've also sent a toybox patch upstream to make that:

  $ ldd toybox
  toybox: cannot open: No such file or directory

Test: manual
Change-Id: If961cdf6f2f2f8d4f6cdfcade9061e49a09d1f01
diff --git a/linker/ldd b/linker/ldd
index 3a0aff9..6bc49b4 100644
--- a/linker/ldd
+++ b/linker/ldd
@@ -10,7 +10,8 @@
 
 [ $# -eq 1 ] || error "usage: ldd FILE"
 
-case `file -L "$1"` in
+what=$(file -L "$1")
+case "$what" in
   *32-bit*)
     linker --list "$1"
     ;;
@@ -18,6 +19,6 @@
     linker64 --list "$1"
     ;;
   *)
-    error "$1: not an ELF file"
+    error "$what"
     ;;
 esac