Missing line number on error

Summary

Missing line number on error for "Missing Element"

Steps to reproduce

Download the project attached and execute bst build extra-cmake-modules.bst

Expected outcome:

Could not find element 'freedesktop-sdk.bst' in elements directory '/data/Projects/Buildstream/test-junctions/elements' on file SomethingBlah.bst

Original outcome:

Could not find element 'freedesktop-sdk.bst' in elements directory '/data/Projects/Buildstream/test-junctions/elements'

• BuildStream version affected: /milestone %BuildStream_v1.x test-junctions.tar.gz

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