fix(ensure): concurrent update args validation

[darkowlzz]

What does this do / why do we need it?

Performs ensure -update arguments validation concurrently.
Related to TODOs:

• TODO(sdboyer) do these concurrently
• TODO(sdboyer) return all errors, not just the first one we encounter

What should your reviewer look out for in this PR?

Concurrent code implementation.

[ibrasho]

Can we print a line to give some context to these errors?

Something like: Invalid arguments passed to ensure -update:

[darkowlzz]

Made the error output similar to manifest validation output with ✗:

$ dep ensure -update github.com/go-yaml/yaml@master github.com/darkowlzz/openurl github.com/sdboyer/deptest/foo
Invalid arguments passed to ensure -update:

  ✗ github.com/sdboyer/deptest/foo is not a project root, try github.com/sdboyer/deptest instead
  ✗ github.com/darkowlzz/openurl is not present in Gopkg.lock, cannot -update it
  ✗ version constraint master passed for github.com/go-yaml/yaml, but -update follows constraints declared in Gopkg.toml, not CLI arguments

update arguments validation failed

[ibrasho]

LGTM

[ibrasho]

I'd call this argsCh.

[ibrasho]

I'd call this errsCh.

[ibrasho]

Apart from the variable names (and they are just an opinion) this LGTM and can be merged.