Make error-handling url configurable

[straker]

In order to get axe-core-npm 4.3.3 released on time, we opted for a less-than-ideal strategy to hard-code the error-handling url in each package. If instead we allowed the url to be configurable, it would make for a cleaner integration.

this.errorUrl = 'https://github.com/dequelabs/axe-core-npm/blob/develop/packages/webdriverjs/error-handling.md';

analyze() {
  try {
  
  } catch(err) {
    throw new Error(`${err.message}. Please see ${this.errorUrl}`)
  }
}
``