ConversionError(InvalidExpression) when using an invalid parse transform

[robertoaloi]

When using an old version of lager that had not been updated to support OTP 24+ line/col numbers, the lager parse transform produced an invalid AST. ELP did not complain during compilation, but eventually logged a ConversionError(InvalidExpression) error.

To repro, logging any message using a version of lager older than d255955 will trigger the ConversionError(InvalidExpr).

We could improve logging in this case, to make the error more obvious to the user. Originally reported in Slack.