More useful detail in log messages

This adds more detail to the output of the `build/check-requires-timestamp` task.

Before:

```bash
$ ./build.py build/check-requires-timestamp
2013-06-11 16:51:28,992 build/check-requires-timestamp: src/ol/source/bingmapssource.js: missing goog.requires: goog.asserts, goog.net.Jsonp, ol.Attribution
2013-06-11 16:51:29,401 pake: build/check-requires-timestamp: 0 unused goog.requires, 3 missing goog.requires
```

After:

```bash
$ ./build.py build/check-requires-timestamp
2013-06-11 16:50:34,046 build/check-requires-timestamp: src/ol/source/bingmapssource.js:92 missing goog.require('goog.asserts')
2013-06-11 16:50:34,046 build/check-requires-timestamp: src/ol/source/bingmapssource.js:40 missing goog.require('goog.net.Jsonp')
2013-06-11 16:50:34,046 build/check-requires-timestamp: src/ol/source/bingmapssource.js:130 missing goog.require('ol.Attribution')
2013-06-11 16:50:34,458 pake: build/check-requires-timestamp: 0 unused goog.requires, 3 missing goog.requires
```