chore: Improve PyCapsule exception handling (#4232)

* Improve pycapsule error handling corner cases

* Handle another corner case

* Simplify err handling code

Author: Skylion007

include/pybind11/pytypes.h

@@ -1829,7 +1829,7 @@ class capsule : public object {
             // guard if destructor called while err indicator is set
             error_scope error_guard;
             auto destructor = reinterpret_cast<void (*)(void *)>(PyCapsule_GetContext(o));
-            if (PyErr_Occurred()) {
+            if (destructor == nullptr && PyErr_Occurred()) {
                 throw error_already_set();
             }
             const char *name = get_name_in_error_scope(o);
@@ -1843,7 +1843,7 @@ class capsule : public object {
             }
         });
 
-        if (!m_ptr || PyCapsule_SetContext(m_ptr, (void *) destructor) != 0) {
+        if (!m_ptr || PyCapsule_SetContext(m_ptr, reinterpret_cast<void *>(destructor)) != 0) {
             throw error_already_set();
         }
     }