py: add variable argument exception constructor function.

Addresses issue #104.

py/objexcept.c

@@ -1,6 +1,7 @@
 #include <stdlib.h>
 #include <stdint.h>
 #include <string.h>
+#include <stdarg.h>
 #include <assert.h>
 
 #include "nlr.h"
@@ -79,6 +80,37 @@ mp_obj_t mp_obj_new_exception_msg_2_args(qstr id, const char *fmt, const char *a
     return o;
 }
 
+mp_obj_t mp_obj_new_exception_msg_varg(qstr id, const char *fmt, ...) {
+    // count number of arguments by number of % signs, excluding %%
+    int n_args = 1; // count fmt
+    for (const char *s = fmt; *s; s++) {
+        if (*s == '%') {
+            if (s[1] == '%') {
+                s += 1;
+            } else {
+                n_args += 1;
+            }
+        }
+    }
+
+    // make exception object
+    mp_obj_exception_t *o = m_new_obj_var(mp_obj_exception_t, void*, n_args);
+    o->base.type = &exception_type;
+    o->id = id;
+    o->n_args = n_args;
+    o->args[0] = fmt;
+
+    // extract args and store them
+    va_list ap;
+    va_start(ap, fmt);
+    for (int i = 1; i < n_args; i++) {
+        o->args[i] = va_arg(ap, void*);
+    }
+    va_end(ap);
+
+    return o;
+}
+
 qstr mp_obj_exception_get_type(mp_obj_t self_in) {
     assert(MP_OBJ_IS_TYPE(self_in, &exception_type));
     mp_obj_exception_t *self = self_in;