Check if hush-cli binary exists before using it

contrib/block_time.pl

@@ -8,6 +8,9 @@ use strict;
 # Given a block time, estimate when it will happen
 my $block      = shift || die "Usage: $0 123";
 my $hush       = "./src/hush-cli";
+unless (-e $hush) {
+    die "$hush does not exist, aborting";
+}
 my $blockcount = qx{$hush getblockcount};
 
 unless ($blockcount = int($blockcount)) {