Catch and log error when writing pages in generate subcommand

Resolves #37

Author: mattt

Sources/swift-doc/Subcommands/Generate.swift

@@ -90,6 +90,8 @@ extension SwiftDoc {
                     let url = outputDirectoryURL.appendingPathComponent(filename)
                     try $0.value.write(to: url, format: format)
                 }
+            } catch {
+                logger.error("\(error)")
             }
         }
     }