Remove error log and replace with debug log

[chillipeper]

Description

Fixes #1910

Status

READY

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• Tests passing
• locally tested in sample instance

Checklist:

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[codecov]

Codecov Report

Merging #1911 (c2750c7) into master (33241f2) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1911      +/-   ##
==========================================
- Coverage   99.35%   99.35%   -0.01%     
==========================================
  Files          83       83              
  Lines        5117     5116       -1     
==========================================
- Hits         5084     5083       -1     
  Misses         33       33              

Impacted Files	Coverage Δ
opsdroid/connector/slack/connector.py	100.00% <100.00%> (ø)

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[FabioRosado]

Thank you for the fix!