fix(httpd): abort processing write request when encountering a precision error.

[StoneYunZhao]

abort processing write request when encountering a precision error.

Reproducing:

curl -X POST http://localhost:8086/write?db=1&rp=2&precision=aa

Expect:

{"error":"invalid precision \"aa\" (use n, u, ms, s, m or h)"}

Got:

{"error":"invalid precision \"aa\" (use n, u, ms, s, m or h)"}
{"error":"database not found: \"1\""}

Describe your proposed changes here.

• CHANGELOG.md updated with a link to the PR (not the Issue)
• Well-formatted commit messages
• Rebased/mergeable
• Tests pass
• Signed CLA (if not already signed)