dist/tools/build_system_sanity_check: handle error by prepending

Handle putting the final error message by prepending the output.
This removes issue with errors being concatenated and gives an
interractive output.

The errors are now send to stderr.

dist/tools/buildsystem_sanity_check/check.sh

@@ -110,19 +110,14 @@ error_on_input() {
     grep '' && return 1
 }
 
+all_checks() {
+    check_not_parsing_features
+    check_not_exporting_variables
+}
+
 main() {
-    local errors=''
-
-    errors+="$(check_not_parsing_features)"
-    errors+="$(check_not_exporting_variables)"
-
-    if [ -n "${errors}" ]
-    then
-        printf 'Invalid build system patterns found by %s:\n' "${0}"
-        printf '%s\n' "${errors}"
-        exit 1
-    fi
-    exit 0
+    all_checks | prepend 'Invalid build system patterns found by '"${0}:"  || error_on_input >&2
+    exit $?
 }