Better documentation of usolve limitations, or possibly error message when input is not upper triangular?

Minimal code:

```
window.onload = function() {
const U = math.matrix([[0.10452846326765346,-120],[-0.9945218953682733,-0]])
const b = math.matrix([-22.163070207475485, -59.419761706007094])

result = math.usolve(U,b)
console.log(result)
}
```

The error I get: `Uncaught Error: Linear system cannot be solved since matrix is singular`.

I entered the very same equation into GNU Octave which was able to solve it: it is no unsolvable system. When switching the lines, it even becomes an upper triangular matrix with nonzero values on the diagonal line. So its determinant is not zero, which implicates that the matrix is not even singular!