Better error messages for `bst track` for cross-junction elements

Summary

Error messages for bst track when junctions are involved do not contain enough information.

Steps to reproduce

junctions/junction-a.bst

kind: junction
sources:
- kind: git
  track: master
  url: upstream:foo/junction-a
  ref: someref12345

org/repo-a.bst

kind: import
depends:
- junctions/junction-a.bst:root-junction-a.bst
sources:
- kind: git
  track: master
  url: upstream:org/repo-a

Contents of foo/junction-a repository

root-junction-a.bst

What is the current bug behavior?

Since the foo/junction-a project does not have a project.conf file, we get the following output when trying to run bst source track org/repo-a.bst:

[--:--:--][        ][ main:core activity                 ] START   Track
[--:--:--][        ][ main:core activity                 ] START   Loading elements
[00:00:00][        ][ main:core activity                 ] FAILURE Loading elements
[00:00:00][        ][ main:core activity                 ] FAILURE Track

Could not find element 'root-junction-a.bst' in elements directory 'elements'

which does not tell us much about the actual problem

What is the expected correct behavior?

Ideally, the error message should say something along the lines of

Junction 'junction-a.bst' does not point to a valid BuildStream project

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