Make all errors in start warnings, actually throw when calling save_payload

[disberd]

This PR tries to fix JuliaPlots/PlotlyJS.jl#482

For consistency, now all errors encountered during start() are just sent as warning and only thrown when calling save_payload.

This should not break precompilation of PlotlyJS but the actual error message will not be thrown in PlotlyJS as a custom savefig implementation is used (which does not call PlotlyKaleido.savefig)

[disberd]

I ended up removing the variable storing the error message and defaulting to @warn instead of error.

Now a more descriptive error is thrown inside save_payload if the process is not running when the function is called (the function will anyway throw with broken pipe error if the startup failed before this PR)

[BeastyBlacksmith]

Looks good to me

[disberd]

Thanks @BeastyBlacksmith. I'll then merge a publish a new non-breaking release to fix PlotlyJS precompilation