ruleguard: use parseError.error field for the returned error (#220)

So we actually return an `*ImportError` instead of `*parseError`
which can't be examined on the caller side.

ruleguard/parser.go

@@ -283,7 +283,7 @@ func (p *rulesParser) parseRuleGroup(f *ast.FuncDecl) (err error) {
 			return
 		}
 		if parseErr, ok := rv.(parseError); ok {
-			err = parseErr
+			err = parseErr.error
 			return
 		}
 		panic(rv) // not our panic

ruleguard/ruleguard_error_test.go

@@ -1,12 +1,36 @@
 package ruleguard
 
 import (
+	"errors"
 	"fmt"
 	"go/token"
 	"strings"
 	"testing"
 )
 
+func TestImportError(t *testing.T) {
+	src := `
+	package gorules
+	import "github.com/quasilyte/go-ruleguard/dsl"
+	func badLock(m dsl.Matcher) {
+		m.Import("foo/nonexisting")
+		m.Match("$x").Where(m["x"].Type.Implements("nonexisting.Iface")).Report("ok")
+	}
+	`
+	e := NewEngine()
+	ctx := &ParseContext{
+		Fset: token.NewFileSet(),
+	}
+	err := e.Load(ctx, "rules.go", strings.NewReader(src))
+	if err == nil {
+		t.Fatal("expected an error, got none")
+	}
+	var importError *ImportError
+	if !errors.As(err, &importError) {
+		t.Fatal("got import that is not ImportError")
+	}
+}
+
 func TestParseFilterFuncError(t *testing.T) {
 	type testCase struct {
 		src string