Error message for numbers that should be strings in arguments could be clearer

```
baseCommand: tail
arguments:
- -n
- +2
- $(input.assembly_identifiers.path)
```
fails with an error because +2 is interpreted as a number instead of a string.
This is really confusing for new users.
```
Object
`....cwl` is
not valid because
tried `CommandLineTool` but
...:14:1: the `arguments` field is not valid because
tried array of but
...:16:5: item is invalid because
- tried string but
the value is not string
- tried Expression but
value is a int but expected a string
```

Could maybe be something.
```
Object
`....cwl` is
not valid because
tried `CommandLineTool` but
...:14:1: the `arguments` field is not valid because
tried array of but
...:16:5: item is invalid because
- tried string but
the value is not string
- tried Expression but
value is a int but expected a string
- '+2' looks like a number, try quoting
```