Clean up unified error handling migration guides and mention anyhow (#2057)

Author: alice-i-cecile

content/learn/migration-guides/0.15-to-0.16.md

@@ -6,4 +6,8 @@ weight = 11
 long_title = "Migration Guide: 0.15 to 0.16"
 +++
 
+The most important changes to be aware of this release are:
+
+- Bevy has reworked its error handling to make it easier to handle `Result`s everywhere. As a result, `Query::single` and friends now return results, rather than panicking.
+
 {{ migration_guides(version="0.16") }}

release-content/0.16/migration-guides/16589_Fallible_systems.md

@@ -1 +1,8 @@
-This change should be pretty much non-breaking, except for users who have implemented their own custom executors. Those users should use `ScheduleSystem` in place of `BoxedSystem<(), ()>` and import the `System` trait where needed. They can choose to do whatever they wish with the result.
+Users who have implemented their own custom executor should use `ScheduleSystem` in place of `BoxedSystem<(), ()>` and import the `System` trait where needed. 
+
+Custom executors should:
+
+- obey the `SystemParamValidationError` returned by `SystemParam::validate_param`, skipping systems as requested
+- use the `default_error_handler` for both validation errors and results returned from systems
+
+See the source code of the first-party Bevy schedule executors for more details.

release-content/0.16/migration-guides/18082_Make_Querysingle_and_friends_return_a_Result.md

@@ -5,3 +5,15 @@
 - Use `unwrap()` or `Ok` destructuring inside of tests.
 
 The old `Query::get_single` (etc) methods which did this have been deprecated.
+
+If you are using `anyhow`, you will experience namespace clashes between Bevy's catch-all `Result` and `anyhow`'s type of the same name.
+Within Bevy-specific projects, you should migrate to use the `bevy::ecs::error::Result` due to its better backtraces, and file requests for any missing functionality.
+For projects that support both Bevy and non-Bevy users, you should define a feature-gated type alias and avoid glob-importing `bevy::prelude`:
+
+```rust
+#[cfg(feature="bevy")]
+type Result = bevy::ecs::error::Result;
+
+#[cfg(not(feature="bevy"))]
+type Result = anyhow::Result;
+```

release-content/0.16/migration-guides/_guides.toml

@@ -167,8 +167,8 @@ areas = ["ECS"]
 file_name = "17602_Encapsulate_cfgfeature__track_location_in_a_type.md"
 
 [[guides]]
-title = "Fallible systems"
-prs = [16589]
+title = "Fallible systems and custom schedule executors"
+prs = [16589, 18541, 18666]
 areas = ["ECS"]
 file_name = "16589_Fallible_systems.md"
 
@@ -221,8 +221,8 @@ areas = ["ECS"]
 file_name = "17671_Isolate_component_registration.md"
 
 [[guides]]
-title = "Make Query::single (and friends) return a Result"
-prs = [18082]
+title = "Unified error handling"
+prs = [18082, 18144]
 areas = ["ECS"]
 file_name = "18082_Make_Querysingle_and_friends_return_a_Result.md"