Bad error message for singular matrix.

[KristofferC]

Downstream issue: JuliaGPU/AMGX.jl#21.

The input matrix in that issue is singular but there seems to be some issue where instead of getting a well-descriptive error message, the more general Fail to get info from cudense is printed.

[mattmartineau]

The error messaging can be improved here on the coarse solve. I'll take a look at that.