Don't panic if site path can't be resolved

Summary:
Throw an explicit error, with rich metadata to help debug from the web log:

• what site?
• init or deploy?

CLI can also benefit of this more explicit error message.

Fixes T1830.

Test Plan:

• HTTP. Fire web server, POST to http://localhost:10206/deploy/foo
• CLI. alkane deploy foo

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1830

Differential Revision: https://devcentral.nasqueron.org/D3010