Unhelpful messages when the manifest deserialization fails

[raymondk]

For eg:

environments:
  - name: local
    network: mainnet

With a manifest like this, the error will look like this:

DEBUG icp: Starting icp-cli version="v0.1.0-beta.1+rev5.dirty-a6057ef" git_sha="a6057efda785212611ef4783c8fc3d791351ccae" command=Project(Show(ShowArgs))
DEBUG icp: Loading project
DEBUG icp: Located icp project in /Users/raymond/tmp/my-motoko
Error: failed to load project

Caused by:
    0: failed to load project manifest
    1: failed to parse manifest at '/Users/raymond/tmp/my-motoko/icp.yaml'
    2: environments: data did not match any variant of untagged enum Item at line [...]

Even though we properly raise an this error

This is because serde ends up swallowing the variant errors....