Make split_string check&throw instead of assert return value

The 2-string version of split_string previously asserted that the split would
result in the right number of substrings. That made it not very useful for
parsing user input, as using it would require parsing the string before passing
it to split_string.

Author: hannes-steffenhagen-diffblue

src/util/string_utils.cpp

@@ -7,6 +7,7 @@ Author: Daniel Poetzl
 \*******************************************************************/
 
 #include "string_utils.h"
+#include "exception_utils.h"
 #include "invariant.h"
 
 #include <cassert>
@@ -109,7 +110,14 @@ void split_string(
   std::vector<std::string> result;
 
   split_string(s, delim, result, strip);
-  CHECK_RETURN(result.size() == 2);
+  if(result.size() != 2)
+  {
+    throw deserialization_exceptiont{"expected string `" + s +
+                                     "' to contain two substrings "
+                                     "delimited by " +
+                                     delim + " but has " +
+                                     std::to_string(result.size())};
+  }
 
   left=result[0];
   right=result[1];