proxy: .rm endpoint should not fail on errors

[shoenig]

e.g.

[p1 tmp] $ curl $GOPROXY/indeed.com/gophers/libconfig/@v/list
v1.0.3
v1.0.4
v2.0.0
v2.0.0-alpha
v2.0.0-alpha2
v2.0.0-alpha2
v2.0.0-alpha3
v2.0.0-alpha4
v2.0.0-alpha4
v2.0.0-alpha5
v2.0.0-alpha5
[p1 tmp] $ 
[p1 tmp] $ 
[p1 tmp] $ curl -XPOST $GOPROXY/indeed.com/gophers/libconfig/@v/v2.0.0-alpha.rm
remove /opt/modprox/zip/indeed.com/gophers/libconfig/v2.0.0-alpha.rm.zip: no such file or directory

This should also continue with attempting to delete the module out of the boltdb, since that is where the list of modules is coming from.