Fix: help: crm help <topic> does not work (#1567)

[nicholasyang2022]

1. When len(args) is 1, should check if it is a topic first.
2. Should check _is_command() or _is_level() before calling help_command().
3. Refine the error message printed when no suitable help entries are found.

Fixes #1567.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 69.76%. Comparing base (e5c5acf) to head (61322a8).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
crmsh/help.py	50.00%	2 Missing ⚠️

Additional details and impacted files

Flag	Coverage Δ
integration	54.57% <50.00%> (-0.01%)	⬇️
unit	52.62% <0.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
crmsh/help.py	88.85% <50.00%> (+0.22%)	⬆️

... and 1 file with indirect coverage changes

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