Exit early if EMAIL envvar is not defined

announce.sh supposes the EMAIL environment variable is set
(since it calls release-notes without setting it.

Exit early with an error message if it's not set.

Change-Id: Ic9f00478dfeca69c4ecbe5fc126105c445c34c4e

README.rst

@@ -134,7 +134,8 @@ announce.sh
 -----------
 
 This script generates an email message ready to be sent to announce
-the release of a library or other tool.
+the release of a library or other tool. Note that the EMAIL environment
+variable needs to be set prior to running the tool.
 
 Example:
 

announce.sh

@@ -33,6 +33,11 @@ REPODIR=$1
 VERSION=$2
 SHORTNAME=`basename $REPODIR`
 
+if [[ -z "$EMAIL" ]]; then
+    echo "ERROR: The EMAIL environment variable is not set."
+    exit 1
+fi
+
 if [[ -z "$VIRTUAL_ENV" ]]; then
     (cd $TOOLSDIR && tox -e venv --notest)
     source $TOOLSDIR/.tox/venv/bin/activate