Improve error handling and update exports (breaking change)

[ecraig12345]

Previously, with continue: true, PGraph would throw an array of errors. Throwing non-Error things is not recommended, and it was especially confusing since what was thrown could vary based on options.

This PR introduces a new class PGraphError which has a message summarizing the other errors, and has a property taskErrors: unknown[] with the original errors. A PGraphError is now thrown any time a task fails, regardless of the continue option. (A regular Error could also be thrown if there's an initial validation error.)

Since this is already a breaking change, I decided to also remove the default export function and remove the pGraph wrapper function. Instead, just import PGraph then do new PGraph().

One other new nice-to-have feature is that the task-level run function is now optional, and PGraph.run() has a new option run?: (taskId: string) => unknown to run a shared function for each task. This makes it possible to reuse the same PGraph initialization to run multiple functions, since calculating the graph could get somewhat expensive if there are many tasks.