Convert exceptions when converting to pdf into an error result

The file processing tries pdf conversion once and keeps going if it
fails. Some errors (e.g. timeouts) are raised via an exception.

Issue: #387

modules/convert/src/main/scala/docspell/convert/extern/ExternConv.scala

@@ -58,6 +58,13 @@ private[extern] object ExternConv {
       }
       .compile
       .lastOrError
+      .attempt
+      .flatMap {
+        case Right(v) =>
+          v.pure[F]
+        case Left(ex) =>
+          handler.run(ConversionResult.failure(ex))
+      }
 
   def readResult[F[_]: Sync: ContextShift](
       blocker: Blocker,