disk: change invalid boot mode error and add test

The previous error message was printing an invalid enum value as a
string, which would cause a panic in the BootMode.String() method.

Update the error in the AddPartitionsForBootMode() function and add a
test.

pkg/disk/partition_table.go

@@ -1097,7 +1097,7 @@ func AddPartitionsForBootMode(pt *PartitionTable, bootMode platform.BootMode) er
 	case platform.BOOT_NONE:
 		return nil
 	default:
-		return fmt.Errorf("invalid boot mode specified: %s", bootMode)
+		return fmt.Errorf("unknown or unsupported boot mode type with enum value %d", bootMode)
 	}
 }
 

pkg/disk/partition_table_test.go

@@ -1199,6 +1199,11 @@ func TestAddPartitionsForBootMode(t *testing.T) {
 			bootMode: platform.BOOT_HYBRID,
 			errmsg:   "error creating BIOS boot partition: unknown or unsupported partition table type: super-gpt",
 		},
+		"bad-bootmode": {
+			pt:       disk.PartitionTable{Type: "gpt"},
+			bootMode: 4,
+			errmsg:   "unknown or unsupported boot mode type with enum value 4",
+		},
 	}
 
 	for name := range testCases {