Log instead of throwing when saving generated sources

Filenames exceeding file system limits shouldn't break the
build if they cannot be saved to disk; they just wouldn't be
available for sourcemaps.
This is a similar issue to
https://github.com/gwtproject/gwt/issues/6014

Bug: #9263
Bug-Link: https://github.com/gwtproject/gwt/issues/9263
Change-Id: Ie4b9f8aaaffaec04640cc92ed3071c0ae85fa35e

dev/core/src/com/google/gwt/dev/SourceSaver.java

diff --git a/dev/core/src/com/google/gwt/dev/SourceSaver.java b/dev/core/src/com/google/gwt/dev/SourceSaver.java
index c0e5896..baed342 100644
--- a/dev/core/src/com/google/gwt/dev/SourceSaver.java
+++ b/dev/core/src/com/google/gwt/dev/SourceSaver.java
@@ -230,8 +230,7 @@
         out.close();
       }
     } catch (IOException e) {
-      log.log(TreeLogger.ERROR, "Fatal error emitting artifact: " + newPath, e);
-      throw new UnableToCompleteException();
+      log.log(TreeLogger.WARN, "Error emitting artifact: " + newPath, e);
     }
   }
 }