loader: clearer error if no junction project.conf

I found the original message confusing - it would talk about wanting a
project at '.', without a frame of reference. After re-reading the code
and junction docs it made more sense, I think this version would save
folks the trouble.

Removing all mention of it, if it's not specified. If it is specified
then explain it a bit more, i.e.

Before:

    Could not find the project.conf file for junction element at
    hello-junction.bst [line 1 column 0]. Expecting a project at path
    'project'

After:

    Could not find the project.conf file in project referred to by
    junction element 'hello-junction.bst'. Was expecting it at path
    'project' in the junction's source.

If no 'path' specified in the junction:

    Could not find the project.conf file in project referred to by
    junction element 'hello-junction.bst'.

1 files changed, 7 insertions, 3 deletions

diff --git a/buildstream/_loader/loader.py b/buildstream/_loader/loader.py
index 9b91e91fe..f7fd3203e 100644
--- a/buildstream/_loader/loader.py
+++ b/buildstream/_loader/loader.py
@@ -570,10 +570,14 @@ class Loader():
                               parent_loader=self)
         except LoadError as e:
             if e.reason == LoadErrorReason.MISSING_PROJECT_CONF:
+                message = (
+                    "Could not find the project.conf file in the project "
+                    "referred to by junction element '{}'.".format(element.name)
+                )
+                if element.path:
+                    message += " Was expecting it at path '{}' in the junction's source.".format(element.path)
                 raise LoadError(reason=LoadErrorReason.INVALID_JUNCTION,
-                                message="Could not find the project.conf file for {}. "
-                                        "Expecting a project at path '{}'"
-                                .format(element, element.path or '.')) from e
+                                message=message) from e
             else:
                 raise