scaler typed config: simplify missing parameter error

[wozniakjan]

@rickbrouwer noticed that #5997 introduced a small side effect with how TypedConfig returns errors for missing trigger metadata parameter names.

error parsing XYZ metadata: unable to set param ["abc"] value

This PR improves the readability of the error by going back to

error parsing XYZ metadata: unable to set param "abc" value

and alternatively listing multiple parameter names as just comma-separated

error parsing XYZ metadata: unable to set param "Abc,abc" value

Checklist

• Tests have been added
• Commits are signed with Developer Certificate of Origin (DCO - learn more)

[SpiritZhou]

I used to think that using brackets was a way to indicate that it can support multiple names. But now it seems that omitting the brackets can make the text more readable. :)