[AIRFLOW-6627] Email with incorrect DAG not delivered

[GuzikJakub]

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Issue link: AIRFLOW-6627

Make sure to mark the boxes below before creating PR: [x]

• Description above provides context of the change

In case of incorrect DAG execution, I did not receive a notification about the erroneous task completion. This was caused by an incorrect encode of the character

I introduced a lock that tries to send an email with an error, adding the exception that when the email fails, it sends information that the logs could not be thrown into the email.

• Commit message/PR title starts with [AIRFLOW-NNNN]. AIRFLOW-NNNN = JIRA ID^*

Link: https://issues.apache.org/jira/browse/AIRFLOW-6627

• Unit tests coverage for changes (not needed for documentation changes)

Not needed

• Commits follow "How to write a good git commit message"
• Relevant documentation is updated including usage instructions.
• I will engage committers as explained in Contribution Workflow Example.

^* For document-only changes commit message can start with [AIRFLOW-XXXX].

[codecov-io]

Codecov Report

Merging #7250 into master will decrease coverage by 0.28%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #7250      +/-   ##
==========================================
- Coverage   85.33%   85.05%   -0.29%     
==========================================
  Files         791      791              
  Lines       40137    40142       +5     
==========================================
- Hits        34252    34143     -109     
- Misses       5885     5999     +114

Impacted Files	Coverage Δ
airflow/models/taskinstance.py	94.71% <66.66%> (-0.25%)	⬇️
airflow/kubernetes/volume_mount.py	44.44% <0%> (-55.56%)	⬇️
airflow/kubernetes/volume.py	52.94% <0%> (-47.06%)	⬇️
airflow/kubernetes/pod_launcher.py	45.25% <0%> (-46.72%)	⬇️
airflow/kubernetes/refresh_config.py	50.98% <0%> (-23.53%)	⬇️
...viders/cncf/kubernetes/operators/kubernetes_pod.py	76.47% <0%> (-21.18%)	⬇️

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[potiuk]

Nice.