The file belchertown.js seems to be missing. That could mean it isn't generated. Check if the file Belchertown/js/belchertown.js.tmpl exists. If it is present, check the configuration in skin.conf. Under [CheetahGenerator] there must me a leaf showing something like this and check if the Belchertown entry is present:
[[ToDate]]
# Reports that show statistics "to date", such as day-to-date,
# week-to-date, month-to-date, etc.
[[[weewx_data]]]
template = json/weewx_data.json.tmpl
[[Belchertown]]
template = js/belchertown.js.tmpl
[[[home]]]
template = index.html.tmpl
[[[about]]]
template = about/index.html.tmpl
[[[graphs]]]
template = graphs/index.html.tmpl
[[[records]]]
template = records/index.html.tmpl
[[[reports]]]
template = reports/index.html.tmpl
[[[pi]]]
template = pi/index.html.tmpl
[[[manifest]]]
encoding = utf8
template = manifest.json.tmpl