Display colliding vehicle name in mapgen failure messages

[PatrikLundell]

Summary

None

Purpose of change

Provide some help in trying to find out why mapgen (and base camp upgrades in particular) fail by providing the name of the first colliding "vehicle" encountered in the error message.

This was prompted by #76627, the save of which was used for testing.

Describe the solution

Change a whole lot of boolean functions to return strings instead, where an empty string corresponds to true/success, and the string otherwise containing the name of the offending vehicle/appliance.
There are a couple of error cases where the cause isn't actually a vehicle/appliance, so the string for those cases make that clear (which results in a weird looking message should these cases actually occur.

Describe alternatives you've considered

Testing

Loaded the save from #76627 (second character) and tried to finish the expansion, resulting in the following error message:

Additional context

It seems the vehicle display name operation prepends a "the" to the name, so I remove the "the" that was in the strings previously.
While looking for "the the" I fixed a number of comments as a by-catch.

Note that the error report only reports on the first clashing vehicle/appliance, so you might have to make a fair number of passes to find them all, but it's better than no indications at all.

[RenechCDDA]

Change a whole lot of boolean functions to return strings instead, where an empty string corresponds to true/success, and the string otherwise containing the name of the offending vehicle/appliance.

I deeply recommend using ret_val instead, similar to #65341. Checking whether the string is empty is not good, among other problems the vehicle's name might be an empty string.

[PatrikLundell]

I'm going to try to understand the ret_val stuff and shift to using it. Thus, this PR is set to draft.
Thanks for the recommendation.

[Maleclypse]

@PatrikLundell I probably should have left conflict resolution to you. I was trying to match what I thought we were moving towards for tripoints but now it looks like I removed a file you were changing. ranged.cpp. My apologies if that is the case.

[PatrikLundell]

If something was lost we'll eventually do it again (and potentially have to do it sooner, if it results in failures to compile). No big deal in either case, but it's nice to be notified, rather than end up with a sense of déja vue later.