Fix false positive when asserting an error pointer

When using a pointer as an error, like in:

```go
type myErr struct {
    code int
}

func (e *myErr) Error() string {
    ...
}

...
err = &myErr{code: 404}
Expect(err).To(HaveOccured())
```

ginkgolinter mistakely detects `err` as not error type.

This PR fixes this issue.

internal/expression/value/value.go

@@ -194,6 +194,10 @@ func IsExprError(pass *analysis.Pass, expr ast.Expr) bool {
 		return interfaces.ImplementsError(actualArgType)
 
 	case *gotypes.Pointer:
+		if interfaces.ImplementsError(t) {
+			return true
+		}
+
 		if tt, ok := t.Elem().(*gotypes.Named); ok {
 			return interfaces.ImplementsError(tt)
 		}

testdata/src/a/haveoccurred/haveoccurred.go

@@ -47,3 +47,38 @@ var _ = Describe("Succeed", func() {
 	})
 
 })
+
+type myError struct {
+	e string
+}
+
+func (e *myError) Error() string {
+	return e.e
+}
+
+func retErr(str string) *myError {
+	return &myError{e: str}
+}
+
+var _ = Describe("pointer error", func() {
+	Context("err pointer var", func() {
+		It("should not trigger warning", func() {
+			err := &myError{e: "err"}
+			Expect(err).To(HaveOccurred())
+		})
+
+	})
+
+	Context("returned value", func() {
+		It("should not trigger warning", func() {
+			err := retErr("err")
+			Expect(err).To(HaveOccurred())
+		})
+	})
+
+	Context("Succeed", func() {
+		It("should not trigger warning", func() {
+			Expect(retErr("err")).ToNot(Succeed())
+		})
+	})
+})