dtc: Make dtc_open_file() die() if unable to open requested file

All current callers of dtc_open_file() immediately die() if it returns
an error.  In a non-interative tool like dtc, it's hard to see what
you could sensibly do to recover from a failure to open an input file
in any case.

Therefore, make dtc_open_file() itself die() if there's an error
opening the requested file.  This removes the need for error checking
at the callsites, and ensures a consistent error message in all cases.
While we're at it, change the rror message from fstree.c when we fail
to open the input directory to match dtc_open_file()'s error message.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

dtc.c

@@ -193,10 +193,6 @@ int main(int argc, char *argv[])
 		bi = dt_from_fs(arg);
 	} else if(streq(inform, "dtb")) {
 		inf = dtc_open_file(arg, NULL);
-		if (!inf)
-			die("Couldn't open \"%s\": %s\n", arg,
-			    strerror(errno));
-
 		bi = dt_from_blob(inf->file);
 	} else {
 		die("Unknown input format \"%s\"\n", inform);