The checkNegativeCache() method existed but was never called, making
negative caching (for failed fetches) completely non-functional.
Failed URLs were being re-fetched on every request.
Add negative cache check at the start of Service.Get() to short-circuit
requests for recently-failed URLs.
Fixes#3
ok sneak.berlin/go/pixa/internal/config
ok sneak.berlin/go/pixa/internal/encurl
ok sneak.berlin/go/pixa/internal/handlers
ok sneak.berlin/go/pixa/internal/imgcache
ok sneak.berlin/go/pixa/internal/middleware
ok sneak.berlin/go/pixa/internal/seal
ok sneak.berlin/go/pixa/internal/session
### Test only (no fix): FAIL
```
# sneak.berlin/go/pixa/internal/imgcache [sneak.berlin/go/pixa/internal/imgcache.test]
internal/imgcache/negative_cache_test.go:96:9: undefined: ErrNegativeCached
internal/imgcache/negative_cache_test.go:97:21: undefined: ErrNegativeCached
FAIL sneak.berlin/go/pixa/internal/imgcache [build failed]
FAIL
```
### Test + fix: PASS
```
ok sneak.berlin/go/pixa/internal/config
ok sneak.berlin/go/pixa/internal/encurl
ok sneak.berlin/go/pixa/internal/handlers
ok sneak.berlin/go/pixa/internal/imgcache
ok sneak.berlin/go/pixa/internal/middleware
ok sneak.berlin/go/pixa/internal/seal
ok sneak.berlin/go/pixa/internal/session
```
Blocking a user prevents them from interacting with repositories, such as opening or commenting on pull requests or issues. Learn more about blocking a user.
5690dc39f4toe651e672aaTest only (no fix): FAIL
Test + fix: PASS