Properly handle nonexistent packages

[andrius-puksta-sensmetry]

Sysand does not work well when asked to add a nonexistent package:

• It gives a cryptic error:

sysand add 'http://example.com'`
      Adding usage: 'http://example.com'
error: Failed to satisfy usage constraints:
requested project(s) alternative nr 0 depends on http://example.com

• Nonexistent project is added to usages in .project.json

sysand env install gives:

sysand env install 'http://example.com'`
error: There is no solution
There is no solution