smartcontract: improve manifest validness errors

It should be clear from error what's wrong with ABI
(specify bad method/event/parameter identifier).

pkg/smartcontract/manifest/abi.go

@@ -2,6 +2,7 @@ package manifest
 
 import (
 	"errors"
+	"fmt"
 	"sort"
 
 	"github.com/nspcc-dev/neo-go/pkg/vm/stackitem"
@@ -53,12 +54,12 @@ func (a *ABI) GetEvent(name string) *Event {
 // IsValid checks ABI consistency and correctness.
 func (a *ABI) IsValid() error {
 	if len(a.Methods) == 0 {
-		return errors.New("ABI contains no methods")
+		return errors.New("no methods")
 	}
 	for i := range a.Methods {
 		err := a.Methods[i].IsValid()
 		if err != nil {
-			return err
+			return fmt.Errorf("method %q/%d: %w", a.Methods[i].Name, len(a.Methods[i].Parameters), err)
 		}
 	}
 	if len(a.Methods) > 1 {
@@ -92,7 +93,7 @@ func (a *ABI) IsValid() error {
 	for i := range a.Events {
 		err := a.Events[i].IsValid()
 		if err != nil {
-			return err
+			return fmt.Errorf("event %q/%d: %w", a.Events[i].Name, len(a.Events[i].Parameters), err)
 		}
 	}
 	if len(a.Events) > 1 {

pkg/smartcontract/manifest/manifest.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"encoding/json"
 	"errors"
+	"fmt"
 	"math"
 
 	ojson "github.com/nspcc-dev/go-ordered-json"
@@ -104,7 +105,7 @@ func (m *Manifest) IsValid(hash util.Uint160) error {
 	}
 	err = m.ABI.IsValid()
 	if err != nil {
-		return err
+		return fmt.Errorf("ABI: %w", err)
 	}
 	err = Groups(m.Groups).AreValid(hash)
 	if err != nil {

pkg/smartcontract/manifest/parameter.go

@@ -2,6 +2,7 @@ package manifest
 
 import (
 	"errors"
+	"fmt"
 	"sort"
 
 	"github.com/nspcc-dev/neo-go/pkg/smartcontract"
@@ -75,7 +76,7 @@ func (p Parameters) AreValid() error {
 	for i := range p {
 		err := p[i].IsValid()
 		if err != nil {
-			return err
+			return fmt.Errorf("parameter #%d/%q: %w", i, p[i].Name, err)
 		}
 	}
 	if len(p) < 2 {