Rework HTTP downloader retry logic

Apply retries as global, config-level option `downloadRetries` so that
it can be applied to any aptly command which downloads objects.

Unwrap `errors.Wrap` which is used in downloader.

Unwrap `*url.Error` which should be the actual error returned from the
HTTP client, catch more cases, be more specific around failures.

man/Makefile

@@ -1,10 +1,12 @@
-all: prepare generate
+RUBYBINPATH=$(shell ruby -r rubygems -e 'puts Gem.user_dir')/bin
 
-prepare:
-	gem install specific_install
-	gem specific_install -l smira/ronn
+all: $(RUBYBINPATH)/ronn generate
+
+$(RUBYBINPATH)/ronn:
+	gem install --user-install specific_install
+	gem specific_install --user-install -l smira/ronn
 
 generate:
-	go run ../_man/gen.go
+	PATH=$(RUBYBINPATH):$(PATH) go run ../_man/gen.go
 
-.PHONY: prepare generate
+.PHONY: generate