Add 2 entries to the FAQ.md

This adds 2 sections:

- “Trakeva Missing Backends” → fix #341
- “Which Dependency Failed?” → fix #345

src/doc/FAQ.md

@@ -28,3 +28,45 @@ There are many use cases,
   node”
   [there](https://github.com/hammerlab/biokepi/blob/master/src/lib/pipeline.ml#L454)).
 
+Trakeva Missing Backends
+------------------------
+
+While configuring Ketrew and trying to start a server it may fail with:
+
+```
+[ketrew: ERROR]
+    The "sqlite" backend was not available at Trakeva's compile time (available
+    backends: [ ...  ])
+```
+
+It means that Trakeva was not compiled with support for the database backend
+you are requesting via the [config-file](src/doc/The_Configuration_File.md).
+
+Solutions:
+
+- Read the documentation about [DB Backends](src/doc/Database_Backends.md).
+- Use another backend (the error message tells which ones are available,
+  between brackets, maybe empty).
+- Install the backend you want:
+    - `opam install postgresql` or `opam install sqlite3` (`opam` will then
+      catch the change and recompile `trakeva` and `ketrew`).
+    - If you are installing `sqlite3` on MacOSX you should read
+      [this](https://github.com/smondet/trakeva#sqlite3-on-macosx).
+
+Which Dependency Failed?
+------------------------
+
+Sometimes you submit a 2000-nodes pipeline, and when it fails you want to see
+the node that “really” failed; i.e. that didn't fail because one of its
+dependencies failed.
+
+Solution: use `(and (is-failed) (not (is-dependency-dead)))` as part of the
+filter in the WebUI.
+
+There is an example ready to try in the Help section:
+
+![Screen Shot Of The Filter's Help Section](https://cloud.githubusercontent.com/assets/617111/12961777/261356d0-d010-11e5-9619-9e02a7efe08b.png)
+
+See also more discussion at issue
+[`#345`](https://github.com/hammerlab/ketrew/issues/345).
+