Add: Check that towns can build roads before generating map.

[PeterN]

Motivation / Problem

Some NewGRFs set introduction dates and flags for road types in such a way that towns cannot build them, or cannot build them until a specific date.

In these circumstances during map generation the game will briefly 'hang' as it tries to repeatly place towns that can never be built, before finally giving up. The delay is larger depending map size and number of towns to build.

Eventually the map generation aborts, and the user is greeted with the message "Map generation aborted... ... no suitable town locations" which does not explain the problem at all.

Description

Instead, before generating the terrain, check if any town-buildable road types are available. If none can be built, immediately abort landscape generation, and show an error message (similar to the warning for vehicles not being available yet.)

This error message explains better what the issue is, and how it can be resolved, including a date if that is required.

Limitations

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR touches english.txt or translations? Check the guidelines
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, game_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker