avoid raising on bad dsn

[mitchellhenke]

closes #194

Previously, we'd raise if the DSN was not in the correct format (for whatever reason). This could be considered a bug, as we avoid raising while trying to send events otherwise.

Related to this, we also raise in these places:

sentry-elixir/lib/sentry/client.ex

Line 226 in f88b9bf

raise ArgumentError, message: ":before_send_event must be an anonymous function or a {Module, Function} tuple"

sentry-elixir/lib/sentry/client.ex

Line 211 in f88b9bf

raise ArgumentError, message: ":after_send_event must be an anonymous function or a {Module, Function} tuple"

For consistency, we should likely avoid raising there as well I think?