Improve error description formatting for validation failure

Instead of printing a literal array (brackets and all!) of errors, print each error on its own indented line.

Author: dfed

apollo-ios-codegen/Sources/ApolloCodegenLib/ApolloCodegen+Errors.swift

@@ -23,7 +23,8 @@ extension ApolloCodegen {
       switch self {
       case let .graphQLSourceValidationFailure(lines):
         return """
-          An error occured during validation of the GraphQL schema or operations! Check \(lines)
+          An error occured during validation of the GraphQL schema or operations! Check:
+              \(lines.joined(separator: "\n    "))
           """
       case .testMocksInvalidSwiftPackageConfiguration:
         return """