charmstore/router: generate http.StatusNotFound appropriately
add params/error.go
more error masking; more error tests; more comments
Merge branch 'v4' into 021-http-status

Conflicts:
	internal/router/router.go
	internal/router/router_test.go
	internal/v4/api.go
rename charm.URL to charm.Reference after merge
fix typo
Merge pull request #50 from rogpeppe/021-http-status

charmstore/router: generate http.StatusNotFound appropriately

This generates messages (and error codes) as expected by the API specification.