Add context to EvalError

[jordemort]

Closes #142

[colindean]

Awesome!

[jordemort]

@colindean This was technically an API break because it changes the constructor of EvalError but I'm not sure it's worth 2.0 on its own

I was going to try and knock out these over the next couple weeks before I tag a new major version:

• Support struct #148
• Can't tell what starlark Builtin cannot be converted to Python #143
• Add API for compiled expressions #12
• Integrate starlib and bazel-skylib #5

If you want this change in a published release sooner LMK and I'll tag a point release for you; I think it's OK to bend the semver rules since I doubt anyone is using the EvalError constructor directly.