Cleanup and update to PHP5 class conventions

- Should throw exceptions on errors that prevent proper `__cosntruct()` execution.
- Should throw exceptions on errors that prevent proper execution of other logic, unless actual error handling is implemented internally to such logic.

Also minimized operation overhead by moving initial node list loading to end of successful login, instead of on every action possible post-login.  (There were checks to determine if upstream requests were required to fulfill this need, but that check itself presents an amount of overhead which can be undesirable in a high performance, high request load situation.)

Additionally, removed superfluous `pve_` from property and method names.  The fact that they are part of the `PVE2_API` class tells us that much of their nature.  The fact that all were in protected/private scope makes the extra characters even more unnecessary.

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NOTE: `diff` isn't very smart about detecting when newlines are added to existing text, so the first several changes (until about line 83 in the old/53 in the new) are less drastic than they appear.  (`git` uses `diff` rather than replicate its functionality because Linus believes in not reinventing wheels he doesn't have to; git is obviously a wheel he had to :grin: )