If an invalid status code is generated from an error we risk panicking. Even if there 
are no potential problems at the moment we should prevent this in the future.

Add safeguards against this.

Sample trace:

```
May 02 06:41:39   minio[52806]: panic: "GET /20180401230655.PDF": invalid WriteHeader code 0
May 02 06:41:39   minio[52806]: goroutine 16040430822 [running]:
May 02 06:41:39   minio[52806]: runtime/debug.Stack(0xc01fff7c20, 0x25c4b00, 0xc0490e4080)
May 02 06:41:39   minio[52806]:         runtime/debug/stack.go:24 +0x9f
May 02 06:41:39   minio[52806]: github.com/minio/minio/cmd.setCriticalErrorHandler.func1.1(0xc022048800, 0x4f38ab0, 0xc0406e0fc0)
May 02 06:41:39   minio[52806]:         github.com/minio/minio/cmd/generic-handlers.go:469 +0x85
May 02 06:41:39   minio[52806]: panic(0x25c4b00, 0xc0490e4080)
May 02 06:41:39   minio[52806]:         runtime/panic.go:965 +0x1b9
May 02 06:41:39   minio[52806]: net/http.checkWriteHeaderCode(...)
May 02 06:41:39   minio[52806]:         net/http/server.go:1092
May 02 06:41:39   minio[52806]: net/http.(*response).WriteHeader(0xc0406e0fc0, 0x0)
May 02 06:41:39   minio[52806]:         net/http/server.go:1126 +0x718
May 02 06:41:39   minio[52806]: github.com/minio/minio/internal/logger.(*ResponseWriter).WriteHeader(0xc032fa3ea0, 0x0)
May 02 06:41:39   minio[52806]:         github.com/minio/minio/internal/logger/audit.go:116 +0xb1
May 02 06:41:39   minio[52806]: github.com/minio/minio/internal/logger.(*ResponseWriter).WriteHeader(0xc032fa3f40, 0x0)
May 02 06:41:39   minio[52806]:         github.com/minio/minio/internal/logger/audit.go:116 +0xb1
May 02 06:41:39   minio[52806]: github.com/minio/minio/internal/logger.(*ResponseWriter).WriteHeader(0xc002ce8000, 0x0)
May 02 06:41:39   minio[52806]:         github.com/minio/minio/internal/logger/audit.go:116 +0xb1
May 02 06:41:39   minio[52806]: github.com/minio/minio/cmd.writeResponse(0x4f364a0, 0xc002ce8000, 0x0, 0xc0443b86c0, 0x1cb, 0x224, 0x2a9651e, 0xf)
May 02 06:41:39   minio[52806]:         github.com/minio/minio/cmd/api-response.go:736 +0x18d
May 02 06:41:39   minio[52806]: github.com/minio/minio/cmd.writeErrorResponse(0x4f44218, 0xc069086ae0, 0x4f364a0, 0xc002ce8000, 0x0, 0x0, 0x0, 0x0, 0x0, 0xc00656afc0)
May 02 06:41:39   minio[52806]:         github.com/minio/minio/cmd/api-response.go:798 +0x306
May 02 06:41:39   minio[52806]: github.com/minio/minio/cmd.objectAPIHandlers.getObjectHandler(0x4b73768, 0x4b73730, 0x4f44218, 0xc069086ae0, 0x4f82090, 0xc002d80620, 0xc040e03885, 0xe, 0xc040e03894, 0x61, ...)
May 02 06:41:39   minio[52806]:         github.com/minio/minio/cmd/object-handlers.go:456 +0x252c
```