:crypto.strong_rand_bytes/1 can throw a `low_entropy` exception

[igalic]

according to its documentation, :crypto.strong_rand_bytes/1 can throw an exception:

May throw exception low_entropy in case the random generator failed due to lack of secure "randomness".

we should at least document that this can happen

there's some options beyond that:

• do/try around :crypto.strong_rand_bytes(16) to optionally return an error
• have an weak_entropy_init/1 helper that can be used on potentially low entropy systems

this kind of issue will generally happens early after startup when not enough entropy hasn't been gathered, or if the system doesn't implement a sensible PRNG (a CSPRNG, that is;)