Move ARPACKException here from LinearAlgebra, fixes #64. (#65)

* Move ARPACKException here from LinearAlgebra, fixes #64.

* Fix test failure on Julia 1.2 due to new eigenvalue sorting.

src/libarpack.jl

@@ -1,6 +1,28 @@
 # This file is a part of Julia. License is MIT: https://julialang.org/license
 
-import LinearAlgebra: BlasInt, ARPACKException
+import LinearAlgebra: BlasInt
+@static if isdefined(LinearAlgebra, :ARPACKException)
+    import LinearAlgebra: ARPACKException
+else
+    struct ARPACKException <: Exception
+        info::BlasInt
+    end
+
+    function Base.showerror(io::IO, ex::ARPACKException)
+        print(io, "ARPACKException: ")
+        if ex.info == -8
+            print(io, "error return from calculation of a real Schur form.")
+        elseif ex.info == -9
+            print(io, "error return from calculation of eigenvectors.")
+        elseif ex.info == -14
+            print(io, string("did not find any eigenvalues to sufficient accuracy. ",
+                "Try with a different starting vector or more Lanczos vectors ",
+                "by increasing the value of ncv."))
+        else
+            print(io, "unspecified ARPACK error: $(ex.info)")
+        end
+    end
+end
 
 ## aupd and eupd wrappers
 

test/runtests.jl

@@ -101,7 +101,7 @@ using Test, LinearAlgebra, SparseArrays, Random
         k = 3
         A = randn(n,n); A = A'A
         B = randn(n,k); B = B*B'
-        @test sort(eigs(A, B, nev = k, sigma = 1.0)[1]) ≈ sort(eigvals(A, B)[1:k])
+        @test sort(eigs(A, B, nev = k, sigma = 1.0)[1]) ≈ sort(eigvals(A, B); by=abs)[1:k]
     end
 end