Add error checking to C style io

src/storm/storage/dd/sylvan/InternalSylvanAdd.cpp

@@ -803,17 +803,27 @@ namespace storm {
         void InternalAdd<DdType::Sylvan, ValueType>::exportToDot(std::string const& filename, std::vector<std::string> const&, bool) const {
             // Open the file, dump the DD and close it again.
             FILE* filePointer = fopen(filename.c_str() , "a+");
+            // fopen returns a nullptr on failure
+            if (filePointer == nullptr) {
+                STORM_LOG_ERROR("Failure to open file: " << filename);
+            } else {
                 this->sylvanMtbdd.PrintDot(filePointer);
                 fclose(filePointer);
             }
+        }
 
         template<typename ValueType>
         void InternalAdd<DdType::Sylvan, ValueType>::exportToText(std::string const& filename) const {
             // Open the file, dump the DD and close it again.
             FILE* filePointer = fopen(filename.c_str() , "a+");
+            // fopen returns a nullptr on failure
+            if (filePointer == nullptr) {
+                STORM_LOG_ERROR("Failure to open file: " << filename);
+            } else {
                 this->sylvanMtbdd.PrintText(filePointer);
                 fclose(filePointer);
             }
+        }
 
         template<typename ValueType>
         AddIterator<DdType::Sylvan, ValueType> InternalAdd<DdType::Sylvan, ValueType>::begin(DdManager<DdType::Sylvan> const& fullDdManager, InternalBdd<DdType::Sylvan> const& variableCube, uint_fast64_t numberOfDdVariables, std::set<storm::expressions::Variable> const& metaVariables, bool enumerateDontCareMetaVariables) const {