cmd/tsdbrelay: accept responses with 2xx code
cmd/tsdbrelay: accept responses with 2xx code

opentsdb may return 204 or 200 on successful PUTs to /api/put. 204 indicates success with no response body, but when the details parameter is specified, a 200 is returned along with a simple response document. tsdbrelay should relay all successful PUTs to bosun.

I am explicitly not changing relayMetadata, since bosun itself appears to only return a 204.

(Also fix a missing format specifier in the verbose() call.)
Skyscanner/bosun,melifaro/bosun,noblehng/bosun,GROpenSourceDev/bosun,bosun-monitor/bosun,leapar/bosun,leapar/bosun,simnv/bosun,alienth/bosun,bookingcom/bosun,Skyscanner/bosun,seekingalpha/bosun,bookingcom/bosun,CheRuisiBesares/bosun,augporto/bosun,noblehng/bosun,CheRuisiBesares/bosun,melifaro/bosun,simnv/bosun,CheRuisiBesares/bosun,augporto/bosun,simnv/bosun,bosun-monitor/bosun,TrentScholl/bosun,GROpenSourceDev/bosun,mhenderson-so/bosun,alienth/bosun,Skyscanner/bosun,augporto/bosun,CheRuisiBesares/bosun,csmith-palantir/bosun,alienth/bosun,alienth/bosun,augporto/bosun,bookingcom/bosun,melifaro/bosun,Skyscanner/bosun,mhenderson-so/bosun,melifaro/bosun,TrentScholl/bosun,GROpenSourceDev/bosun,mhenderson-so/bosun,GROpenSourceDev/bosun,TrentScholl/bosun,GROpenSourceDev/bosun,leapar/bosun,bosun-monitor/bosun,simnv/bosun,noblehng/bosun,csmith-palantir/bosun,bosun-monitor/bosun,alienth/bosun,seekingalpha/bosun,alienth/bosun,csmith-palantir/bosun,noblehng/bosun,bosun-monitor/bosun,seekingalpha/bosun,seekingalpha/bosun,bookingcom/bosun,augporto/bosun,simnv/bosun,CheRuisiBesares/bosun,bookingcom/bosun,seekingalpha/bosun,noblehng/bosun,mhenderson-so/bosun,Skyscanner/bosun,GROpenSourceDev/bosun,bosun-monitor/bosun,melifaro/bosun,TrentScholl/bosun,mhenderson-so/bosun,Skyscanner/bosun,TrentScholl/bosun,leapar/bosun,simnv/bosun,mhenderson-so/bosun,csmith-palantir/bosun,CheRuisiBesares/bosun,augporto/bosun,melifaro/bosun,leapar/bosun,noblehng/bosun,TrentScholl/bosun,csmith-palantir/bosun,bookingcom/bosun,csmith-palantir/bosun,leapar/bosun,seekingalpha/bosun