Chandan Singh authored Jun 06, 2018 and Tristan Van Berkom committed Jun 06, 2018

It can be confusing, especially to new BuildStream users, that the CLI
expects targets to be specified relative to element-path and not the
current directory. Previously, the CLI would give a generic message
stating that the file could not be found but it was not obvious that it
was looking in the `element-path` directory.

Explicitly print the element-path in the summary. Also, try to check if
the specified element exists in the elements directory and print a hint
to use the element-path relative paths if that's the case.

Fixes #396.

This is is also related to #341. This commit aims to tackle that issue
by trying to educate users about element-path.