See https://github.com/golang/go/wiki/CodeReviewComments#indent-error-flow

While Client.Recover and Client.RecoverWithContext are mostly meant to
be internal methods called by methods with the same name on Hub, they
are part of the exported API on Client.

This commit avoids a runtime panic when those methods are called with a
nil *EventHint.

This eliminates some code duplication.

Note that it is important to keep the call to recover in the body of the
function itself, because the Go spec defines that the return value of
recover is (always) nil if it was not called directly by a deferred
function.

The previous behavior was to silently stop panics without reporting to
Sentry when the return value from recover() was not an error or a
string.

Runtime panics are specified to implement the predefined error
interface, and string is probably the second most common type passed to
panic() calls, still calls like panic(42) should not pass silently.

This fix converts any other type to string using Go's standard
formatting rules and reports the message to Sentry.