Add error::what tag to store identifier strings

This should replace the dynamic messages generated at throw site.

Issue #219

src/error.cc

@@ -20,6 +20,8 @@
 
 #include "error.h"
 
+#include <boost/exception/get_error_info.hpp>
+
 namespace scram {
 
 const char Error::kPrefix_[] = "scram error: ";
@@ -28,6 +30,10 @@ Error::Error(std::string msg)
     : msg_(std::move(msg)),
       thrown_(kPrefix_ + msg_) {}
 
-const char* Error::what() const noexcept { return thrown_.c_str(); }
+const char* Error::what() const noexcept {
+  if (const char* const* msg = boost::get_error_info<error::what>(*this))
+    return *msg;
+  return thrown_.c_str();
+}
 
 }  // namespace scram

src/error.h

@@ -24,10 +24,22 @@
 #include <exception>
 #include <string>
 
+#include <boost/exception/error_info.hpp>
 #include <boost/exception/exception.hpp>
 
 namespace scram {
 
+namespace error {
+
+/// 'what' identifier for errors deriving from std::exception.
+///
+/// @note This should not be the error message to be printed for users.
+///       It is the optional *identifier* string.
+///       By default, boost::diagnostic_information_what is used.
+using what = boost::error_info<struct tag_what, const char*>;
+
+}  // namespace error
+
 /// The Error class is the base class
 /// for common exceptions specific to the SCRAM code.
 class Error : virtual public std::exception, virtual public boost::exception {