Improve parsing to datetime and add error handling

[JonnyWong16]

Description

• Improve parsing utils.toDatetime() and add error handling.
• Fix utils.millisecondToHumanstr() for values greater than days and negative values.

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated the docstring for new or existing methods
• I have added tests when applicable

[simonc56]

* Improve parsing `utils.toDatetime()` and add error handling.

With python 3.9 this PR not only improves but changes the utils.toDatetime() return.

Before PR, toDatetime() returns a local time (UTC+2 here) :

>>> datetime.datetime.fromtimestamp(1593721093)
datetime.datetime(2020, 7, 2, 22, 18, 13)

After PR, toDatetime() returns a UTC time :

>>> datetime.datetime.utcfromtimestamp(0) + datetime.timedelta(seconds=1593721093)
datetime.datetime(2020, 7, 2, 20, 18, 13)

Do I open a new issue ?

Refs :

• Sync command doesn't respect timezone anymore Taxel/PlexTraktSync#1601
• Fix datetime returning UTC #1262

[JonnyWong16]

Hmm, yes it should be reverted to fromtimestamp().