Error message logged for every emitted trace

[rdacquisto]

Describe the bug
Every time a trace is emitted, I am seeing a null error in the logs. The cause of this log message is

dd-trace-js/packages/dd-trace/src/exporters/agent/writer.js

Line 78 in d830ae6

this._prioritySampler.update(JSON.parse(res).rate_by_service)

this._prioritySampler is undefined. The e from the catch block is

TypeError: Cannot read property 'update' of undefined.

The variable e isn't exposed, and the null err variable is returned. err Cannot have a value and reach

dd-trace-js/packages/dd-trace/src/exporters/agent/writer.js

Line 80 in d830ae6

log.error(err)

.

Environment

• Operation system: Linux
• Node version: 10.15.3
• Tracer version: 0.15.4 (seen since at least 0.15.1. The stack only recently was made visible.)
• Agent version: Latest (Installed today)

[rochdev]

The null part was fixed in #716. The actual error will also be fixed soon.