idmap2: move commands to Result<Unit>

Change the signature of the idmap2 commands (Create, Dump, ...) to
return Result<Unit> instead of bool. This removes the need to pass in an
ostream for error messages: instead, those messages are part of the
returned Result.

Consolidate error messages: texts in Error objects should not be
prefixed with "error:", that is the responsibility of the outer-most
caller (i.e. main()).

Test: make idmap2_tests
Change-Id: I074881b3d1982ea8f4be5752161ac74b14fcba95
diff --git a/cmds/idmap2/idmap2d/Idmap2Service.cpp b/cmds/idmap2/idmap2d/Idmap2Service.cpp
index fa94414..e03a9cc 100644
--- a/cmds/idmap2/idmap2d/Idmap2Service.cpp
+++ b/cmds/idmap2/idmap2d/Idmap2Service.cpp
@@ -104,8 +104,7 @@
   std::ifstream fin(idmap_path);
   const std::unique_ptr<const IdmapHeader> header = IdmapHeader::FromBinaryStream(fin);
   fin.close();
-  std::stringstream dev_null;
-  *_aidl_return = header && header->IsUpToDate(dev_null);
+  *_aidl_return = header && header->IsUpToDate();
 
   // TODO(b/119328308): Check that the set of fulfilled policies of the overlay has not changed