add dc_get_last_error() (#2788)

* add dc_get_last_error()

* make clippy happy

* simplify block_on() call

deltachat-ffi/deltachat.h

@@ -2414,6 +2414,22 @@ dc_array_t* dc_get_locations                (dc_context_t* context, uint32_t cha
 void        dc_delete_all_locations         (dc_context_t* context);
 
 
+/**
+ * Get last error string.
+ *
+ * This is the same error string as logged via #DC_EVENT_ERROR,
+ * however, using this function avoids race conditions
+ * if the failing function is called in another thread than dc_get_next_event().
+ *
+ * @memberof dc_context_t
+ * @param context The context object.
+ * @return Last error or an empty string if there is no last error.
+ *     NULL is never returned.
+ *     The returned value must be released using dc_str_unref() after usage.
+ */
+char* dc_get_last_error (dc_context_t* context);
+
+
 /**
  * Release a string returned by another deltachat-core function.
  * - Strings returned by any deltachat-core-function