Collects all linear algebra errors into linalg/exceptions.jl

In #2962 I found that linalg/blas.jl uses exceptions that are only defined within the LAPACK module. It would make more sense to take the exceptions out of linalg/lapack.jl so that they can be referenced more easily in the linear algebra functions.

Currently this doesn't quite seem to work; I get an error when I try to compile this:
```
LoadError("sysimg.jl",159,LoadError("linalg.jl",154,LoadError("linalg/blas.jl",30,ErrorException("in yield: Scheduler not defined"))))
```