Add error codes for signing error checks

include/jwt/exceptions.hpp

@@ -0,0 +1,35 @@
+#ifndef CPP_JWT_EXCEPTIONS_HPP
+#define CPP_JWT_EXCEPTIONS_HPP
+
+#include <new>
+
+namespace jwt {
+
+/**
+ */
+class MemoryAllocationException final: public std::bad_alloc
+{
+public:
+  /**
+   * Construct MemoryAllocationException from a
+   * string literal.
+   */
+  template <size_t N>
+  MemoryAllocationException(const char(&msg)[N])
+    : msg_(&msg[0])
+  {
+  }
+
+  virtual const char* what() const noexcept override
+  {
+    return msg_;
+  }
+
+private:
+  const char* msg_ = nullptr;
+};
+
+
+} // END namespace jwt
+
+#endif