Improve error message on firewall controller version not found.

[Gerrit91]

Sometimes looks like this:

Last Operation:
TIME                    STATE   PROGRESS        DESCRIPTION                                                                                                       
2021-08-02T09:24:21Z    Error   32% [Reconcile] Flow "Shoot cluster reconciliation" encountered task errors: [task "Waiting                                     
                                                until shoot infrastructure has been reconciled" failed: admission webhook                                       
                                                "validation.metal.provider.extensions.gardener.cloud" denied the request: controllerVersion:                    
                                                Required value: firewall controller version:438931e was not found in available versions: [{89c80cb              
                                                https://images.metal-stack.io/firewall-controller/pull-requests/82-dns-based-egress-policies/firewall-controller
                                                0xc00092e9e0} {4e19541                                                                                           
                                                https://images.metal-stack.io/firewall-controller/pull-requests/109-fix-reload/firewall-controller 0xc00092ea00}
                                                {v1.0.5 https://images.metal-stack.io/firewall-controller/v1.0.5/firewall-controller 0xc00092ea10} {v1.0.9      
                                                https://images.metal-stack.io/firewall-controller/v1.0.9/firewall-controller 0xc00092ea60} {v1.0.10             
                                                https://images.metal-stack.io/firewall-controller/v1.0.10/firewall-controller 0xc00092eac0} {v1.1.2             
                                                https://images.metal-stack.io/firewall-controller/v1.1.2/firewall-controller 0xc00092eaf0}]] Operation will be  
                                                retried. 

Or:

Error: [POST /v1/cluster][500] updateCluster default  admission webhook "validation.metal.provider.extensions.gardener.cloud" denied the request: controllerVersion: Required value: firewall controller version:43
8931e was not found in available versions: [{89c80cb https://images.metal-stack.io/firewall-controller/pull-requests/82-dns-based-egress-policies/firewall-controller 0xc000929d10} {92777f9 https://images.metal-s
tack.io/firewall-controller/pull-requests/82-dns-based-egress-policies/firewall-controller 0xc000929d20} {4e19541 https://images.metal-stack.io/firewall-controller/pull-requests/109-fix-reload/firewall-controlle
r 0xc000929d40} {v1.0.5 https://images.metal-stack.io/firewall-controller/v1.0.5/firewall-controller 0xc000929d60} {v1.0.9 https://images.metal-stack.io/firewall-controller/v1.0.9/firewall-controller 0xc000929d8
0} {v1.0.10 https://images.metal-stack.io/firewall-controller/v1.0.10/firewall-controller 0xc000929da0} {v1.1.2 https://images.metal-stack.io/firewall-controller/v1.1.2/firewall-controller 0xc000929db0}] (500)

[Gerrit91]

Error message now looks like:

could not process: controllerVersion: Required value: firewall controller version \"foo\" was not found in available versions: [2d64a16 v1.2.2 v1.3.0 v2.0.0 v2.0.1 v2.0.2 v2.0.3 v2.0.4 v2.0.5]"