Add prefix to namespace (#297)

## tl;dr

- Add prefix to namespace

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **New Features**
- Enhanced error messages for namespace validation to include the
invalid input.
	- Updated output format of the namespace to include a "xmtpd-" prefix.

- **Bug Fixes**
	- Improved clarity of error messages related to namespace validation.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

pkg/db/pgx.go

@@ -71,7 +71,8 @@ func isValidNamespace(namespace string) error {
 	// PostgreSQL identifiers must start with a letter or underscore
 	if !allowedNamespaceRe.MatchString(namespace) {
 		return fmt.Errorf(
-			"namespace must start with a letter or underscore and contain only letters, numbers, and underscores",
+			"namespace must start with a letter or underscore and contain only letters, numbers, and underscores. Instead is %s",
+			namespace,
 		)
 	}
 	return nil

pkg/utils/namespace.go

@@ -1,6 +1,8 @@
 package utils
 
 import (
+	"fmt"
+
 	ethcrypto "github.com/ethereum/go-ethereum/crypto"
 	"github.com/xmtp/xmtpd/pkg/config"
 )
@@ -11,5 +13,5 @@ func BuildNamespace(options config.ServerOptions) string {
 		[]byte(options.Contracts.NodesContractAddress),
 	)
 
-	return HexEncode(hash)[:12]
+	return fmt.Sprintf("xmtpd_%s", HexEncode(hash)[:12])
 }