core: return explicit caption if tests fail to construct context

The previous error was very generic, making it hard to quickly tell from
the test output that the error was during context initialization.

terraform/context_test.go

@@ -151,7 +151,7 @@ func testContext2(t *testing.T, opts *ContextOpts) *Context {
 
 	ctx, err := NewContext(opts)
 	if err != nil {
-		t.Fatalf("err: %s", err)
+		t.Fatalf("failed to create test context\n\n%s\n", err)
 	}
 
 	return ctx