Replace some profile exceptions with preconditions

Misconfigured profiles must fall back to a consistent state, this
is a programming error.

- provider.pool: fall back to default pool (should always be there)
- provider.preset: why would one remove a preset?
- host.endpointProtocols: .ovpn with no remotes shouldn't get this far

Passepartout/Sources/ApplicationError.swift

@@ -30,12 +30,6 @@ enum ApplicationError: Error {
 
     case missingCredentials
     
-    case providerPool
-    
-    case providerPreset
-    
-    case hostEndpoints
-    
     case missingCA
 
     case emptyRemotes

Passepartout/Sources/Model/Profiles/HostConnectionProfile.swift

@@ -52,9 +52,7 @@ class HostConnectionProfile: ConnectionProfile, Codable, Equatable {
     }
     
     func generate(from configuration: TunnelKitProvider.Configuration, preferences: Preferences) throws -> TunnelKitProvider.Configuration {
-        guard !parameters.endpointProtocols.isEmpty else {
-            throw ApplicationError.hostEndpoints
-        }
+        precondition(!parameters.endpointProtocols.isEmpty)
         
         // XXX: copy paste, error prone
         var builder = parameters.builder()

Passepartout/Sources/Model/Profiles/ProviderConnectionProfile.swift

@@ -40,7 +40,7 @@ class ProviderConnectionProfile: ConnectionProfile, Codable, Equatable {
     }
 
     var pool: Pool? {
-        return infrastructure.pool(for: poolId)
+        return infrastructure.pool(for: poolId) ?? infrastructure.pool(for: infrastructure.defaults.pool)
     }
 
     var presetId: String {
@@ -107,10 +107,10 @@ class ProviderConnectionProfile: ConnectionProfile, Codable, Equatable {
     
     func generate(from configuration: TunnelKitProvider.Configuration, preferences: Preferences) throws -> TunnelKitProvider.Configuration {
         guard let pool = pool else {
-            throw ApplicationError.providerPool
+            preconditionFailure("Nil pool?")
         }
         guard let preset = preset else {
-            throw ApplicationError.providerPreset
+            preconditionFailure("Nil preset?")
         }
 
 //        assert(!pool.numericAddresses.isEmpty)