Add retry logic to findElement.

[jeffdav]

• Added maxRetries and retryInterval to session object in case anyone wants to change defaults.
• Added withRetry = true param to all the various public findElement methods.
• Added withRetry without default param to the internal findElement method that does the actual work so future overloads don't accidentally forget to pass the value through.
• Changed private findElement workhorse to internal and unified the Element.findElement methods to call the single implementation in Session. The only difference between the two code paths was the eventual URL that got constructed, and this removes a lot of duplicate code.
• Made a few error messages more clear.

Fixes WIN-496

[jeffdav]

(I'm not sure this autolink thing is working.)

[tristanlabelle]

(I'm not sure this autolink thing is working.)

Maybe it's not setup on this particular repo 🤔

[tristanlabelle]

The structure of the code is looking great but I think should use a different API shape.

[tristanlabelle]

A couple more comments but I think this is the functionality we want! Nice work