refactor: simplify formatMessage internal logic

[legobeat]

This reduces the nesting and branching-complexity of formatMessage in an attempt to make it more readable.

Also changes formatMessage to return [message:string, isString:bool] rather than [message:string, notString:bool] to remove the double-negation inside encodeString and update methods.

Based on and includes:

• 33

[emn178]

merge this PR and close previous?

[legobeat]

@emn178 Thanks for following up! Your call if you prefer them to be merged individually or as one, thought I'd give you the option to separate the functional change from the refactor :)