Add an option to throw an exception from Source.readString decoding errors

[fzhinkin]

Currently, Source.readString can only emit a replacement character when ill-formed UTF-8 code units are encountered during decoding.

A similar stdlib function, ByteArray.decodeToString, however, can throw an exception in such cases if a user specifies the corresponding option (throwOnInvalidSequence).

We should consider providing an API consistent with stdlib.

[kpavlov]

Perhaps we could consider separating reading and decoding? #487