blas: bounds check panic messages do not match netlib/blas

[kortschak]

Running the matrix/mat64 TestMul using the netlib CGO blas causes a trivial failure resulting from a difference in messages for bounds check failures; netlib/blas is more informative.

$ CGO_LDFLAGS="-L/usr/local -lopenblas" go test -tags cblas -run TestMul
--- FAIL: TestMul (0.08s)
	dense_test.go:836: expected runtime panic for nil data slice: got "blas: index of c out of range"
	dense_test.go:836: expected runtime panic for nil data slice: got "blas: index of c out of range"
	dense_test.go:836: expected runtime panic for nil data slice: got "blas: index of c out of range"
	dense_test.go:836: expected runtime panic for nil data slice: got "blas: index of c out of range"
	dense_test.go:836: expected runtime panic for nil data slice: got "blas: index of c out of range"
	dense_test.go:836: expected runtime panic for nil data slice: got "blas: index of c out of range"
FAIL

To fix this we need to add an additional name parameter to the checkMatrix* helpers so that the correct message can be panicked on. This is generally useful anyway.