Add Logging for plist parse errors

This is useful to debug errors that occur because of bad plists sent by
clients. Usually in this case, one receives a not-very-useful error such
as `HTTP status 400 (Bad Request): <opaque error>`.

 ## Test Plan

1. Patch `mdmb` so that it sends bad checkins
```
diff --git a/internal/device/mdm.go b/internal/device/mdm.go
index 33a4c50..cda3d69 100644
--- a/internal/device/mdm.go
+++ b/internal/device/mdm.go
@@ -185,10 +185,7 @@ func (c *MDMClient) connect(client *http.Client, connReq interface{}) error {
                return errors.New("device not enrolled")
        }

-       plistBytes, err := plist.Marshal(connReq)
-       if err != nil {
-               return err
-       }
+       plistBytes := []byte("aoeuaoeuaoeu")
```

2. Run
```
$mdmb -uuids $uuid devices-connect
```

3. Observe more verbose NanoMDM log:
```
2022/07/21 15:52:43 level=info handler=checkin-command trace_id=1af087fdc6de5381 msg=command report results err=HTTP status 400 (Bad Request): decoding command results: parse error: EOF: raw body: aoeuaoeuaoeu
```

mdm/checkin.go

@@ -149,6 +149,9 @@ func (w *checkinUnmarshaller) UnmarshalPlist(f func(interface{}) error) error {
 func DecodeCheckin(rawMessage []byte) (message interface{}, err error) {
 	w := &checkinUnmarshaller{raw: rawMessage}
 	err = plist.Unmarshal(rawMessage, w)
+	if err != nil {
+		err = &ParseError{Err: err, Body: rawMessage}
+	}
 	message = w.message
 	return
 }

mdm/command.go

@@ -35,6 +35,7 @@ func DecodeCommandResults(rawResults []byte) (results *CommandResults, err error
 	results = new(CommandResults)
 	err = plist.Unmarshal(rawResults, results)
 	if err != nil {
+		err = &ParseError{Err: err, Body: rawResults}
 		return
 	}
 	results.Raw = rawResults
@@ -58,7 +59,7 @@ func DecodeCommand(rawCommand []byte) (command *Command, err error) {
 	command = new(Command)
 	err = plist.Unmarshal(rawCommand, command)
 	if err != nil {
-		return
+		return nil, &ParseError{Err: err, Body: rawCommand}
 	}
 	command.Raw = rawCommand
 	if command.CommandUUID == "" || command.Command.RequestType == "" {

mdm/mdm.go

@@ -5,6 +5,7 @@ import (
 	"context"
 	"crypto/x509"
 	"errors"
+	"fmt"
 )
 
 // Enrollment represents the various enrollment-related data sent with requests.
@@ -59,3 +60,16 @@ func (r *Request) Clone() *Request {
 	*r2 = *r
 	return r2
 }
+
+type ParseError struct {
+	Err  error
+	Body []byte
+}
+
+func (e *ParseError) Unwrap() error {
+	return e.Err
+}
+
+func (e *ParseError) Error() string {
+	return fmt.Sprintf("parse error: %s: raw body: %v", e.Err, string(e.Body))
+}