Include more error context in deduction failures

[spenczar]

More liberal use of errors.Wrap gives more complete output, like this:

% dep init
Unable to cache code.justin.tv/release/trace
sm.DeduceProjectRoot: unable to deduce repository and source type for "code.justin.tv/release/trace": unable to read metadata: unable to fetch raw metadata: failed HTTP request to URL "http://code.justin.tv/release/trace?go-get=1": Get http://code.justin.tv/release/trace?go-get=1: dial tcp: lookup code.justin.tv: no such host

This could be improved, readability-wise, but the first step is to preserve the error information up to the caller.

Fixes #556.

[sdboyer]

So, I'm unsure about this. We don't want to double-wrap these errors, and while it's more convenient to wrap them in the central location that sourceGateway provides, my understanding is that it's more useful/better practice to wrap them immediately where they're generated, at the boundaries our code - no?

[spenczar]

@sdboyer I haven't heard that advice. I just use errors.Wrap to build the stack of errors so it can be printed back in a way that makes the error clear... Are you saying you'd prefer this error?

% dep init
Unable to cache code.justin.tv/release/trace
Get http://code.justin.tv/release/trace?go-get=1: dial tcp: lookup code.justin.tv: no such host

[sdboyer]

@spenczar i'm honestly not sure about it myself - @davecheney, did i read this somewhere, or am I pulling this out of my ass - is it suboptimal to errors.Wrap() multiple times?

I think just the dial error may be a bit lower-level than we're looking for, but...well, honestly, I mostly just want us to be consistent.

[sdboyer]

I'm gonna call this fine as incremental progress, and click the big green button 😄