Improve snap build script: check snapcraft exit status, write checksum

Author: Lee Miller

.buildbot/snap/build.sh

@@ -1,7 +1,9 @@
-#!/bin/sh
+#!/bin/bash
 
-cd packages && snapcraft
+pushd packages && snapcraft || exit 1
 
-cd ..
+popd
 mkdir -p ../out
 mv packages/pybitmessage*.snap ../out
+cd ../out
+sha256sum pybitmessage*.snap > SHA256SUMS