error.c: add new error handling API functions; ref #2837

- mrb_clear_error(): clear error status of mrb_state
- mrb_check_error(): check if error caused in the previous API

Note that `mrb_check_error` clears error status, so if you call
`mrb_check_error` more than once, latter calls will return FALSE.

include/mruby/error.h

@@ -74,6 +74,15 @@ mrb_break_value_set(struct RBreak *brk, mrb_value val)
 #define mrb_break_proc_get(brk) ((brk)->proc)
 #define mrb_break_proc_set(brk, p) ((brk)->proc = p)
 
+/**
+ * Error check
+ *
+ */
+/* clear error status in the mrb_state structure */
+MRB_API void mrb_clear_error(mrb_state *mrb);
+/* returns TRUE if error in the previous call; internally calls mrb_clear_error() */
+MRB_API mrb_bool mrb_check_error(mrb_state *mrb);
+
 /**
  * Protect
  *

src/error.c

@@ -649,6 +649,24 @@ mrb_print_error(mrb_state *mrb)
 #endif
 }
 
+/* clear error status in the mrb_state structure */
+MRB_API void
+mrb_clear_error(mrb_state *mrb)
+{
+  mrb->exc = NULL;
+}
+
+/* returns TRUE if error in the previous call; internally calls mrb_clear_error() */
+MRB_API mrb_bool
+mrb_check_error(mrb_state *mrb)
+{
+  if (mrb->exc) {
+    mrb_clear_error(mrb);
+    return TRUE;
+  }
+  return FALSE;
+}
+
 void
 mrb_init_exception(mrb_state *mrb)
 {