Add errors & warning summary section to output

diffoscope result may contain different errors, adding errors section at bottom/top of output which summarize all errors occurs during diffoscope operation and how to fix them (increase related --max option, install missing tool, open bug at diffoscope/upstream-tool), examples:

Errors:

• missing tool '[X] not available in path. Falling back to binary comparison.' such as: #112 (closed)

• Command failed with exit code: ed8be4ad

• binary diff for supported file-type/file-ext without comparator usage (recognizes fail?)

• "​truncated by diffoscope" messages #144

• max messages: Reached max container depth , Max HTML report size reached Max text output size reached Max diff block lines reached

Warnings/Notices:

• Format-specific differences are supported for [X] but no file-specific differences were detected; falling back to a binary diff.

• no comparator used at all (not supported file-type/file-ext) such as: #106 (closed) #116 (closed)