It looks like sometimes diff files cannot be unarchived.
The result of inflate() should at least contain the diff
header even for diffs created from two identical files but
for some reason it appears to be empty.
This commit stops assuming that the downloaded diff
file can be read without errors.
In was used as eps in meters in some cases and eps in mercator in others.
Its value was hard to justify and so were its use cases. We're better off
with less epsilons for now.
Also renamed Metres to Meters in the mercator code.