Small CLI fixes

[Flix6x]

This PR combines a bug fix with several tiny improvements to CLI functions, and the introduction of a developer util decorator.

The bug is this (reporting a successful flexmeasures db-ops dump when in fact it failed):

Backing up 127.0.0.1/flexmeasures-db database
/bin/sh: line 1: pg_dump: command not found
db dump successful: saved to pgbackup_flexmeasures-db_2022-11-01-1510.dump

Likewise for flexmeasures db-ops restore.

The timeit decorator can be added to to CLI functions (or any function, really) to print out the run time of that function. Is this something you'd consider adding as a default for all CLI commands?

[coveralls]

Pull Request Test Coverage Report for Build 3436497061

• 4 of 17 (23.53%) changed or added relevant lines in 4 files are covered.
• 3 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.02%) to 65.095%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
flexmeasures/cli/db_ops.py	0	2	0.0%
flexmeasures/cli/data_delete.py	1	4	25.0%
flexmeasures/utils/coding_utils.py	2	10	20.0%

Files with Coverage Reduction	New Missed Lines	%
flexmeasures/cli/data_delete.py	1	28.07%
flexmeasures/cli/db_ops.py	2	41.46%

Totals
Change from base Build 3436490383:	-0.02%
Covered Lines:	6559
Relevant Lines:	9463

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💛 - Coveralls

[nhoening]

Great.

I haven't decided if I want to add timeit to all commands. Actually, they take an embarrassingly long time (due to Flask's startup time, and maybe also timely beliefs). Not sure I want to know :)