properly handle errors in r1cs backend

[eightfilms]

#100 revealed that in src/backends/r1cs/snarkjs.rs that there are a few places where errors are left unhandled - these should not be silenced like in that PR, but should be properly handled.

Related comments:
#100 (comment)
#100 (comment)

[katat]

Nice spots. I think they are mostly related to file IO errors. We could create some error types in noname to wrap them up for the 3rd party library.