fix plugin error propagation (#830)

#### What this PR does / why we need it:

- If the plugin is called via `go run` (like in the tests), this command
adds an implicit error status line, which
  must be handled in the plugin adapter to find the correct error json.
- The plugin might provide error output. This error output must be
separated from the formal error status
- As long as we don't have a formal way to get to the stderr stream from
the context, the error output is added
  to the error message

cmds/subcmdplugin/cmds/cmd_test.go

@@ -35,6 +35,21 @@ var _ = Describe("subcmdplugin", func() {
 			env.Cleanup()
 		})
 
+		Context("error handling", func() {
+			It("provides error", func() {
+				var buf bytes.Buffer
+
+				ExpectError(env.CatchOutput(&buf).Execute("group", "demo", "--version=error")).To(MatchError("error processing plugin command command: demo error"))
+			})
+
+			It("provides error and error outputp", func() {
+				var buf bytes.Buffer
+
+				ExpectError(env.CatchOutput(&buf).Execute("group", "demo", "--version=error this is an error my friend")).To(MatchError(`error processing plugin command command: demo error: with stderr
+this is an error my friend`))
+			})
+		})
+
 		Context("local help", func() {
 			It("shows group command help", func() {
 				var buf bytes.Buffer

cmds/subcmdplugin/cmds/demo/cmd.go

@@ -2,6 +2,8 @@ package demo
 
 import (
 	"fmt"
+	"os"
+	"strings"
 
 	// bind OCM configuration.
 	_ "github.com/open-component-model/ocm/pkg/contexts/ocm/plugin/ppi/config"
@@ -30,5 +32,12 @@ type command struct {
 
 func (c *command) Run(cmd *cobra.Command, args []string) error {
 	fmt.Printf("demo command called with arguments %v (and version option %s)\n", args, c.version)
+	if strings.HasPrefix(c.version, "error") {
+		msg := strings.TrimSpace(c.version[5:])
+		if len(msg) != 0 {
+			fmt.Fprintf(os.Stderr, "this is an error my friend\n")
+		}
+		return fmt.Errorf("demo error")
+	}
 	return nil
 }

pkg/contexts/ocm/plugin/cache/exec.go

@@ -5,6 +5,9 @@ import (
 	"fmt"
 	"io"
 	"os/exec"
+	"strings"
+
+	"github.com/mandelsoft/goutils/errors"
 
 	"github.com/open-component-model/ocm/pkg/common/accessio"
 	"github.com/open-component-model/ocm/pkg/contexts/ocm/plugin/ppi/cmds"
@@ -29,18 +32,33 @@ func Exec(execpath string, config json.RawMessage, r io.Reader, w io.Writer, arg
 	err := cmd.Run()
 	if err != nil {
 		var result cmds.Error
-		var msg string
-		data := stderr.Bytes()
+		var cerr error
+		data := strings.TrimSpace(string(stderr.Bytes()))
 		if len(data) == 0 {
-			msg = err.Error()
+			cerr = errors.New(err.Error())
 		} else {
-			if err := json.Unmarshal(stderr.Bytes(), &result); err == nil {
-				msg = result.Error
+			// handle implicit error output from go run
+			i := strings.LastIndex(data, "\n")
+			for i > 0 && (i == len(data)-1 || strings.HasPrefix(data[i+1:], "exit status")) {
+				data = data[:i]
+				i = strings.LastIndex(data, "\n")
+			}
+			if err := json.Unmarshal([]byte(data), &result); err == nil {
+				cerr = errors.New(result.Error)
 			} else {
-				msg = fmt.Sprintf("[%s]", string(stderr.Bytes()))
+				if err := json.Unmarshal([]byte(data[i+1:]), &result); err == nil {
+					cerr = errors.New(result.Error)
+					// TODO pass effective stderr from CLI
+					data = strings.TrimSpace(data[:i])
+					if len(data) > 0 {
+						cerr = fmt.Errorf("%w: with stderr\n%s", cerr, data)
+					}
+				} else {
+					cerr = fmt.Errorf("[%s]", data)
+				}
 			}
 		}
-		return nil, fmt.Errorf("%s", msg)
+		return nil, cerr
 	}
 	if l, ok := stdout.(*accessio.LimitedBuffer); ok {
 		if l.Exceeded() {

pkg/contexts/ocm/plugin/plugin.go

@@ -421,7 +421,7 @@ func (p *pluginImpl) Command(name string, reader io.Reader, writer io.Writer, cm
 		return errors.ErrNotFound("command", name)
 	}
 
-	defer finalize.FinalizeWithErrorPropagationf(&rerr, "error processing plugin command call %s", name)
+	defer finalize.FinalizeWithErrorPropagation(&rerr)
 
 	var f vfs.File