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.

(cherry picked from commit d3a3b3aff376e759d821d38d9620b1c21c499e6b)

core/math/basis.cpp

@@ -788,7 +788,7 @@ Basis::operator String() const {
 Quat Basis::get_quat() const {
 
 #ifdef MATH_CHECKS
-	ERR_FAIL_COND_V_MSG(!is_rotation(), Quat(), "Basis must be normalized in order to be casted to a Quaternion. Use get_rotation_quat() or call orthonormalized() instead.");
+	ERR_FAIL_COND_V_MSG(!is_rotation(), Quat(), "Basis must be normalized in order to be casted to a Quaternion. Use get_rotation_quat() or call orthonormalized() if the Basis contains linearly independent vectors.");
 #endif
 	/* Allow getting a quaternion from an unnormalized transform */
 	Basis m = *this;