fix: replaced an assert to an exception in CSkipSet inside Add

assert can be triggered if Add() is called without validation of SkipSet conditions

src/util/skip_set.cpp

@@ -5,10 +5,13 @@
 #include <util/skip_set.h>
 
 #include <logging.h>
+#include <stdexcept>
 
 bool CSkipSet::Add(uint64_t value)
 {
-    assert(!Contains(value));
+    if (Contains(value)) {
+        throw std::runtime_error(strprintf("%s: trying to add an element that can't be added", __func__));
+    }
 
     if (auto it = skipped.find(value); it != skipped.end()) {
         skipped.erase(it);

src/util/skip_set.h

@@ -5,7 +5,7 @@
 #include <serialize.h>
 #include <unordered_set>
 
-// This datastructure keeps efficiently all indexes and have a strict limit for used memory
+// This data structure keeps efficiently all indexes and have a strict limit for used memory
 // So far as CCreditPool is built only in direction from parent block to child
 // there's no need to remove elements from CSkipSet ever, only add them
 class CSkipSet {
@@ -19,8 +19,12 @@ class CSkipSet {
     {}
 
     /**
-     * adding value that already exist in CKipSet will cause `assert`.
+     * `Add` returns true if element has been added correctly and false if
+     * capacity is depleted.
      *
+     * `Add` should not be called if the element has been already added.
+     * Use `Contains` to check if the element is here
+     * Adding existing value will cause an exception
      */
     [[nodiscard]] bool Add(uint64_t value);