Fix help message for usage of `err` variable in `or {}` block

[ArtemkaKun]

error(E0050): cannot pass `os.IOError` to parameter with `string` type
 --> xml.sp:7:45:48
   |
 7 |    file := os.read_file("foo.xml") or { panic(err) }
   |                                               ^^^ expected: string, got: os.IOError

note: parameter has `string` type
 --> /home/yuart/Projects/spawn/y/builtin/panic.sp:24:14:22
    |
 24 | pub fn panic(s string) -> never {
    |              ^^^^^^^^ (1) parameter declared here as `string`

help: `os.IOError` can be converted to `string` with `.str()`
 --> xml.sp:7:48:48
   |
 7 |    file := os.read_file("foo.xml") or { panic(err.str()) }
   |                                                  ++++++ add `.str()` call to convert `os.IOError` to `string`

Help proposes to use .str() but it should propose to use .msg(), since err doesn't have a method .str() implemented