Do not eagerly check for existence in FilePayload

This is a TOCTOU violation and FilePayload.getInput already propagates
this.  This commit allows external callers like jclouds-cli to
introspect on the exception type, returning a more friendly error
message in some situations.

core/src/main/java/org/jclouds/io/payloads/FilePayload.java

@@ -34,7 +34,7 @@ public class FilePayload extends BasePayload<File> {
    public FilePayload(File content) {
       super(content);
       getContentMetadata().setContentLength(content.length());
-      checkArgument(checkNotNull(content, "content").exists(), "file must exist: " + content);
+      checkNotNull(content, "content");
    }
 
    /**