Make get_key_material_file fail more verbosely

It turns out, there are a lot of possible reasons for fopen to fail.
Let's share which reason we failed for today.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes openzfs#12410

lib/libzfs/libzfs_crypto.c

@@ -486,7 +486,7 @@ get_key_material_file(libzfs_handle_t *hdl, const char *uri,
 		ret = errno;
 		errno = 0;
 		zfs_error_aux(hdl, dgettext(TEXT_DOMAIN,
-		    "Failed to open key material file"));
+		    "Failed to open key material file: %s"), strerror(ret));
 		return (ret);
 	}