test: add function die () and have use of it in add_email_corpus ()

Added die() function to test-lib.sh with the following first use of it:

If notmuch new fails during email corpus addition the database is
most probably inexistent or broken and the added corpus would be
unusable while running single tests, giving misleading failures
("only" full 'make test' cleans out old corpus).

test/test-lib.sh

@@ -251,6 +251,16 @@ trap_signal () {
 	exit $code
 }
 
+die () {
+	_exit_common
+	exec >&6
+	say_color error '%-6s' FATAL
+	echo " $*"
+	echo
+	echo "Unexpected exit while executing $0."
+	exit 1
+}
+
 GIT_EXIT_OK=
 # Note: TEST_TMPDIR *NOT* exported!
 TEST_TMPDIR=$(mktemp -d "${TMPDIR:-/tmp}/notmuch-test-$$.XXXXXX")
@@ -544,7 +554,7 @@ add_email_corpus ()
 	cp -a $TEST_DIRECTORY/corpus.mail ${MAIL_DIR}
     else
 	cp -a $TEST_DIRECTORY/corpus ${MAIL_DIR}
-	notmuch new >/dev/null
+	notmuch new >/dev/null || die "'notmuch new' failed while adding email corpus"
 	cp -a ${MAIL_DIR} $TEST_DIRECTORY/corpus.mail
     fi
 }