Fix #12594: Give descriptive error when company takeover fails due to vehicle limit

[2TallTyler]

Motivation / Problem

As per #12594, company takeovers that fail due to vehicle limit do not explain why beyond an error, "Can't buy company..."

Description

Clarify the reason with an existing error string and clarify the comment. Despite its broad name, MayCompanyTakeOver() only checks vehicle limits.

Closes #12594.

Limitations

Does not strictly need backporting, but no languages are changed so I don't see a reason to exclude it.

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