Add option to continue from generalization on error

[tyekx]

As I was importing europe, I ran out of disk space after the pbf import was completed. I really did not want to restart a 4hr process, so I made some changes to the structure of the flow. Then I ran out of space during optimization, restarting that process failed due to indices already existing, so they also got an IF NOT EXISTS statement.

I figured maybe other people could get in my shoes, so here is the PR, feel free to reject it.

Example usage:

$ ./imposm import -connection <connection> -mapping mapping.yaml -optimize -deployproduction -postprocess
[2025-12-30T20:49:11+01:00] 0:00:00 [step] Starting: Imposm
[2025-12-30T20:49:11+01:00] 0:00:00 [step] Starting: Creating generalized tables
[2025-12-30T20:49:11+01:00] 0:00:00 [step] Starting: Generalizing osm_aerialway_linestring into osm_aerialway_linestring_gen_z12
[...]