[MC-185156] Worldgen settings are not validated and can cause crashes

Some parameters are not validated and this can lead to a crash. This bug report is meant to list all those cases.

Parameter	Value	Result
`noise.size_horizontal` `noise.size_vertical`	0	Divide by zero crash-2020-05-21_21.32.22-client.txt
~~`noise.size_horizontal`~~ ~~`noise.size_vertical`~~	~~< 0~~	~~Negative array size crash-2020-05-22_16.09.23-server.txt~~
~~`noise.height`~~	~~< 1~~	~~Bound must be positive crash-2020-05-22_15.41.08-server.txt~~
~~`noise.height`~~	~~> 256~~	~~Index out of bounds crash-2020-05-22_15.46.11-server.txt~~
~~`structures.*.spacing`~~	0	~~Divide by zero crash-2020-05-22_16.41.13-server.txt~~

Condition	Result
~~`structures.*.spacing` <= `separation`~~	~~Bound must be positive crash-2020-05-22_22.35.48-server.txt~~
`noise.height` < 9 and not a void biome	Bound must be positive crash-2020-05-22_21.42.07-server.txt
Unknown (related to `spacing`)	String index out of range crash-2020-05-29_15.15.05-client.txt Reproduce: string_index_range.json

_{This list is incomplete. Please help by commenting and attaching a crash report.}