* Remove all of the errors under core. Their purpose is now served by errors, and they were almost entirely unused. The remaining uses were switched to errors.
* Remove errors.NotSupportedError. It was used in only one place (ca.go), and that usage is more appropriately a ServerInternal error.
This patch removes all usages of the `core.XXXError` and almost all usages of `probs` outside of the WFE and VA and replaces them with a unified internal error type. Since the VA uses `probs.ProblemDetails` quite extensively in challenges, and currently stores them in the DB I've saved this change for another change (it'll also require a migration). Since `ProblemDetails` should only ever be exposed to end-users all of its related logic should be moved into the `WFE` but since it still needs to be exposed to the VA and SA I've left it in place for now.
The new internal `errors` package offers the same convenience functions as `probs` does as well as a new simpler type testing method. A few small changes have also been made to error messages, mainly adding the library and function name to internal server errors for easier debugging (i.e. where a number of functions return the exact same errors and there is no other way to distinguish which method threw the error).
Also adds proper encoding of internal errors transferred over gRPC (the current encoding scheme is kept for `core` and `probs` errors since it'll be ideally be removed after we deploy this and follow-up changes) using `grpc/metadata` instead of the gRPC status codes.
Fixes#2507. Updates #2254 and #2505.
Currently services will pass both `core.XXXError` and `probs.XXX` type errors across the gRPC layer. In the future (#2505) we intend to stop passing `probs.XXX` type errors across this layer but for now we need to support them until that change is landed. This patch takes the easiest path to allow this by encoding the `probs.ProblemDetails` to JSON and storing it in the gRPC error body so that it can be passed around.
Fixes#2497.
Roland Bracewell Shoemaker
Jacob Hoffman-Andrews