Instrustment handlers for logging/error handling

This refactors a bit of the http handler code.
Moves error handling for handler functions to a generic handler.
This also has a side-effect of being able to propagate errors from the
provider to send the correct status code, provided the error type
matches a pre-defined interface.

vendor/github.com/cpuguy83/strongerrors/defs.go

@@ -0,0 +1,84 @@
+package strongerrors
+
+// ErrNotFound signals that the requested object doesn't exist
+type ErrNotFound interface {
+	NotFound()
+}
+
+// ErrInvalidArgument signals that the user input is invalid
+type ErrInvalidArgument interface {
+	InvalidArgument()
+}
+
+// ErrConflict signals that some internal state conflicts with the requested action and can't be performed.
+// A change in state should be able to clear this error.
+type ErrConflict interface {
+	Conflict()
+}
+
+// ErrUnauthorized is used to signify that the user is not authorized to perform a specific action
+type ErrUnauthorized interface {
+	Unauthorized()
+}
+
+// ErrUnauthenticated is used to indicate that the caller cannot be identified.
+type ErrUnauthenticated interface {
+	Unauthenticated()
+}
+
+// ErrUnavailable signals that the requested action/subsystem is not available.
+type ErrUnavailable interface {
+	Unavailable()
+}
+
+// ErrForbidden signals that the requested action cannot be performed under any circumstances.
+// When a ErrForbidden is returned, the caller should never retry the action.
+type ErrForbidden interface {
+	Forbidden()
+}
+
+// ErrSystem signals that some internal error occurred.
+// An example of this would be a failed mount request.
+type ErrSystem interface {
+	System()
+}
+
+// ErrNotModified signals that an action can't be performed because it's already in the desired state
+type ErrNotModified interface {
+	NotModified()
+}
+
+// ErrAlreadyExists is a special case of ErrNotModified which signals that the desired object already exists
+type ErrAlreadyExists interface {
+	AlreadyExists()
+}
+
+// ErrNotImplemented signals that the requested action/feature is not implemented on the system as configured.
+type ErrNotImplemented interface {
+	NotImplemented()
+}
+
+// ErrUnknown signals that the kind of error that occurred is not known.
+type ErrUnknown interface {
+	Unknown()
+}
+
+// ErrCancelled signals that the action was cancelled.
+type ErrCancelled interface {
+	Cancelled()
+}
+
+// ErrDeadline signals that the deadline was reached before the action completed.
+type ErrDeadline interface {
+	DeadlineExceeded()
+}
+
+// ErrExhausted indicates that the action cannot be performed because some resource is exhausted.
+type ErrExhausted interface {
+	Exhausted()
+}
+
+// ErrDataLoss indicates that data was lost or there is data corruption.
+type ErrDataLoss interface {
+	DataLoss()
+}