Include section name and parameter name (if available) in config errors.
Added tests for this.
adding aadding badding t.hadding t/xaNONEXISTENT: No such file or directoryabt.ht/xa: No such file or directoryNONEXISTENT: No such file or directorybt.ht/xaNONEXISTENT: No such file in rev ce18e5bc5cd3at/xbt.h% -I/-X with relative path should workbt.ht/xt/xt/x