build: fix download script for i686

[braydonf]

fix download script for i686 and make sure there is an error if the arch is unknown

closes: bitpay/bitcore#1423 (comment)

[kreshendo]

When will this be merged?

[ScriptProdigy]

Any word on if this will ever be merged? Would solve a lot of problems for us ubuntu/mac/vagrant users! Its been approved, why was the merge not processed?

vagrant@vagrant-ubuntu-trusty-32:~$ npm install -g bitcore-node
\
> bitcore-node@3.1.3 preinstall /home/vagrant/.nvm/versions/node/v4.8.4/lib/node_modules/bitcore-node
> ./scripts/download

Downloading bitcoin: https://github.com/bitpay/bitcoin/releases/download/v0.12.1-bitcore-4/
npm ERR! Linux 3.13.0-119-generic
npm ERR! argv "/home/vagrant/.nvm/versions/node/v4.8.4/bin/node" "/home/vagrant/.nvm/versions/node/v4.8.4/bin/npm" "install" "-g" "bitcore-node"
npm ERR! node v4.8.4
npm ERR! npm  v2.15.11
npm ERR! code ELIFECYCLE

npm ERR! bitcore-node@3.1.3 preinstall: `./scripts/download`
npm ERR! Exit status 22
npm ERR!
npm ERR! Failed at the bitcore-node@3.1.3 preinstall script './scripts/download'.
npm ERR! This is most likely a problem with the bitcore-node package,
npm ERR! not with npm itself.
npm ERR! Tell the author that this fails on your system:
npm ERR!     ./scripts/download
npm ERR! You can get information on how to open an issue for this project with:
npm ERR!     npm bugs bitcore-node
npm ERR! Or if that isn't available, you can get their info via:
npm ERR!
npm ERR!     npm owner ls bitcore-node
npm ERR! There is likely additional logging output above.

npm ERR! Please include the following file with any support request:
npm ERR!     /home/vagrant/npm-debug.log
vagrant@vagrant-ubuntu-trusty-32:~$ uname -i
i686

[metaspartan]

Let's keep making it known! Approve this PR Bitcore devs!!