Some early users of margin plots noticed a bug. If data associated with
a tag for any run was missing, an error message appeared noting that
data was missing for that tag. The error message gave no indication of
which runs were missing information. Sometimes, the behavior is
intended. For instance, some run might not log a certain scalar value.

This change now offers users a more nuanced message when margin plot
logic is missing information for certain tags within a run. The message
notes specific runs and then lists the specific tags without data for
those runs. The message is placed under a collapsible so that the user
can chose to not see the message if the behavior is intended.