Remove Error::kPrefix_ static string

The SCRAM specific identifier is redundant.
Boost exception reporting dynamically prints the exception type.

Issue #219

src/error.cc

@@ -24,16 +24,12 @@
 
 namespace scram {
 
-const char Error::kPrefix_[] = "scram error: ";
-
-Error::Error(std::string msg)
-    : msg_(std::move(msg)),
-      thrown_(kPrefix_ + msg_) {}
+Error::Error(std::string msg) : msg_(std::move(msg)) {}
 
 const char* Error::what() const noexcept {
   if (const char* const* msg = boost::get_error_info<error::what>(*this))
     return *msg;
-  return thrown_.c_str();
+  return msg_.c_str();
 }
 
 }  // namespace scram

src/error.h

@@ -73,15 +73,10 @@ class Error : virtual public std::exception, virtual public boost::exception {
   /// Sets the error messages.
   ///
   /// @param[in] msg  The error message.
-  void msg(std::string msg) {
-    msg_ = std::move(msg);
-    thrown_ = kPrefix_ + msg_;
-  }
+  void msg(std::string msg) { msg_ = std::move(msg); }
 
  private:
-  static const char kPrefix_[];  ///< Prefix specific to SCRAM.
   std::string msg_;  ///< The error message.
-  std::string thrown_;  ///< The message to throw with the prefix.
 };
 
 /// For input/output related errors.