Avoid crashing when slots are missing

[Joey79100]

TEN tries to render lots of things without checking first if the required data is there first. Most common example recently: a missing object in the WAD, such as bars sprites. The consequence is a plain crash, with no information to help users.

When possible, checks should be done: if some required data is missing:

• A warning should appear in TENLog, so the user knows what they need to fix.
• Rendering of this element should be skipped, to avoid the crash.

Here is a list of slots that do not already have such checks, and could use some (with no particular order). Feel free to amend. I just did the few I found from running a "converted" TRNG level with missing sprite objects, and didn't look much further.

Moveables

Sprites

• [x] Bar graphics
• [x] Helical lasers
• [x] Electricity arks
• [ ] "SP_DEF" particles (?)
• [x] "Particles"
• [x] Splashes
• [x] Bubbles
• [x] Drips
• [x] Shockwaves
• [x] Blood
• [x] Underwater dust
• [x] Snow
• [x] Rain
• [x] Smoke
• [x] "Sparks particles" (flare sparks, ricochets sparks, friction sparks, electric sparks, electric cleaner sparks)
• [x] Explosions
• [ ] "Simple particles" (snowmobile snow, speedboat foam)
• [x] Footprints

Other

• [x] Textures folder contents (if the Engine folder is missing the Textures folder and its content, the game will simply not start)

[Joey79100]

Just a did a few on branch warning_not_crashes with 4af6f4999

For some reason Github won't list my branch when trying to link it manually from the issue, and I didn't think to mention the issue in the commit's comment.