Fix a bug causing budget reports to produce errors.

Now the budget report does not give errors any more,
*provided that* -X <commodity> is used (equivalently,
a commodity must be chosen in the dialog).

This is a workaround for this bug in Ledger:

http://bugs.ledger-cli.org/show_bug.cgi?id=378

Apparently, Ledger sometimes produces an error if
the budget involves multiple commodities and amounts
equal to zero.
Fix the same bug in BUDGET_FORMAT.

Also, reverse the sign of the “Remaining” field,
and use “na” instead of zero for undefined percentage.
Improve the alignment of the last line.
Display “0” in the empty fields of budget reports.

Display “0” instead of leaving a field empty,
in the Budgeted and Remaining columns.