detect script improvements; Django => Python/Django; stderr on no detect

bin/detect

@@ -1,8 +1,14 @@
 #!/usr/bin/env bash
-# bin/name <build-dir>
+# bin/detect <build-dir>
 
 BUILD_DIR=$1
-[ -f $BUILD_DIR/requirements.txt ] || exit 1 # fail fast if no requirements.txt
 
-# 'Django' if there is a [mysite]/settings.py file present; otherwise 'Python'
-ls $BUILD_DIR/**/settings.py &> /dev/null && echo Django || echo Python
+function abort() {
+  echo $1 1>&2
+  exit 1
+}
+
+[ -f $BUILD_DIR/requirements.txt ] || abort "fatal: Not a Python app (no requirements.txt)"
+
+# 'Python/Django' if there is a [mysite]/settings.py file present; otherwise 'Python'
+ls $BUILD_DIR/**/settings.py &> /dev/null && echo Python/Django || echo Python