Improve Basis::get_quaternion error message

The previous error message incorrectly suggested that any Basis could be fixed by calling get_rotation_quation() or orthonormalize(). This PR points out that only a valid rotation Basis can be fixed in this way.

core/math/basis.cpp

@@ -763,7 +763,7 @@ Basis::operator String() const {
 
 Quaternion Basis::get_quaternion() const {
 #ifdef MATH_CHECKS
-	ERR_FAIL_COND_V_MSG(!is_rotation(), Quaternion(), "Basis must be normalized in order to be casted to a Quaternion. Use get_rotation_quaternion() or call orthonormalized() instead.");
+	ERR_FAIL_COND_V_MSG(!is_rotation(), Quaternion(), "Basis must be normalized in order to be casted to a Quaternion. Use get_rotation_quaternion() or call orthonormalized() if the Basis contains linearly independent vectors.");
 #endif
 	/* Allow getting a quaternion from an unnormalized transform */
 	Basis m = *this;