Parse out page number in underfull vbox warnings

[thomwiggers]

LaTeX prints out the pages it is typesetting when errors occur. For example, see the following snippet from my document's compile log:

[2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
Underfull \vbox (badness 2080) has occurred while \output is active [13]

This indicates that on page [13] in the output PDF the Underfull warning was triggered. This information is very useful in tracking down where these warnings are coming from, because underfull Vboxen aren't strictly related to line numbers or input files.