mapcache_seed aborts but gives no error code indicating that seeding has failed

[tomhel]

In its current state mapcache_seed will abort a seeding if a tile failed to be retrived from the wms. It will however give no indication (error code) that seeding failed.

When seeding a cache containing several million of tiles, it might not matter much if few tiles fail. At least it does not in my case. When seeding for several weeks there is bound the be some timeouts. Of cource, i understand that there are cases when it does indeed matter.

What i propose is to add some configurable options:

• ignore failed tiles, yes/no
• threshold of how many tiles is allowed to fail before aborting.

The attached patch adds the following non-configurable functionality to mapcache_seed.c

• ignore tiles that failed to be retrieved from wms as long as the number of failed tiles is less than 1% of what has currently been seeded, otherwise abort.
• mapcache_seed will return an error code indicating if tiles failed to be fetched but was ignored (below 1%), another one if seeding was aborted because of to many failed tiles, a third if seeding was aborted by some other error.

[tomhel]

adds the error handling explained in ticket