update should fail when conflicting versions are requested

[jdhuntington]

Scenario:

• Package a inherits from foo and bar.
• foo requests webpack v1.
• bar requests webpack v2.

Expected

package-inherit update prints "unresolvable conflict" and exits with status code != 0 and does not modify any package.json files.

Actual

• package-inherit update completes successfully.
• package-inherit check fails