Improve handling for gamedata user error

[nosoop]

Should properly address #58.

If we ever support 64-bit just remove the corresponding PluginFailMessage entries. :^)

[nosoop]

Compiles and tests fine in Windows x64; just waiting on AM to come back up for the workflow's sake.

[nosoop]

Forgot to rebase. Really fixed now.

[nosoop]

Adjusted based on feedback.

it may be fine to just trust that loadgameconfig handles fileexists already

okay so both windows / linux do report missing file on initial load

[SM] Error parsing gameconfig file "addons\sourcemod\gamedata\tf2.attributes.txt":
[SM] Error 1 on line 0, col 0: Stream failed to open
[SM] Exception reported: Unable to open tf2.attributes: File could not be opened: The system cannot find the file specified.

but unloading + loading afterwards with the file still missing does

[SM] Exception reported: Could not initialize call to CEconItemSchema::GetItemDefinition

[nosoop]

LGTM. We ball