d1b7dff094 return custom error from `Network::from_str` (Noah)
Pull request description:
Fix#1292
Had some time so got this out of the way.
ACKs for top commit:
apoelstra:
ACK d1b7dff094
tcharding:
ACK d1b7dff094
Tree-SHA512: f6566f4df74c697cc3d84eca4e4c45bb5da9e86fe32e5f6e257a3e8c3e22fc375f5f307c7f96edd4536a80c1d1c13535f6073a1a093911468abb015040c2888b