Incorrect error message when unable to snap all input coordinates

[mjjbell]

Issue

Fixes #5845

In cases where we are unable to find a phantom node for an input
coordinate, we return an error indicating which coordinate failed.

This would always refer to the coordinate with index equal to the
number of valid phantom nodes found.

We fix this by instead returning the first index for which a
phantom node could not be found.

Tasklist

• CHANGELOG.md entry (How to write a changelog entry)
• add tests (see testing documentation
• review
• adjust for comments
• cherry pick to release branch

[danpat]

Suggestion: I think this could be done with std::mismatch, which I think helps clarify intent a bit.

auto mismatch = std::mismatch(phantom_nodes.begin(), phantom_nodes.end(), 
                              coordinates.begin(), coordinates.end(),
                              [](const auto &phantom_node, const auto &coordinate) {
                                return phantom_node.first.input_location == coordinate;
                              });
std::size_t missing_index = std::distance(phantom_nodes.begin(), mismatch.first);

[mjjbell]

Nice. Yes, a lot clearer.