add file_getprop() to updater
Add a function to read a property from a ".prop"-formatted file
(key=value pairs, one per line, ignore # comment lines and blank
lines). Move ErrorAbort to the core of edify; it's not specific to
updater now that errors aren't stored in the app cookie.
diff --git a/edify/expr.h b/edify/expr.h
index 671b499..d2e7392 100644
--- a/edify/expr.h
+++ b/edify/expr.h
@@ -118,5 +118,9 @@
// strings it contains.
char** ReadVarArgs(State* state, int argc, Expr* argv[]);
+// Use printf-style arguments to compose an error message to put into
+// *state. Returns NULL.
+char* ErrorAbort(State* state, char* format, ...);
+
#endif // _EXPRESSION_H