seal: Fix and improve error reporting (#6773)

* seal: Rework ext_transfer, ext_instantiate, ext_call error handling

* Deny calling plain accounts (must use transfer now)
* Return proper module error rather than ad-hoc strings
* Return the correct error codes from call,instantiate (documentation was wrong)
* Make ext_transfer fallible again to make it consistent with ext_call

* seal: Improve error messages on memory access failures

* seal: Convert contract trapped to module error

* seal: Add additional tests for transfer, call, instantiate

These tests verify that those functions return the error types
which are declared in its docs.

* Make it more pronounced that to_execution_result handles trap_reason

* Improve ReturnCode docs

* Fix whitespace issues in wat files

* Improve ReturnCode doc

* Improve ErrorOrigin doc and variant naming

* Improve docs on ExecResult and ExecError

* Encode u32 sentinel value as hex

* with_nested_context no longer accepts an Option for trie

* Fix successful typo

* Rename InvalidContractCalled to NotCallable

substrate/frame/contracts/fixtures/set_rent.wat

@@ -1,5 +1,5 @@
 (module
-	(import "env" "ext_transfer" (func $ext_transfer (param i32 i32 i32 i32)))
+	(import "env" "ext_transfer" (func $ext_transfer (param i32 i32 i32 i32) (result i32)))
 	(import "env" "ext_set_storage" (func $ext_set_storage (param i32 i32 i32)))
 	(import "env" "ext_clear_storage" (func $ext_clear_storage (param i32)))
 	(import "env" "ext_set_rent_allowance" (func $ext_set_rent_allowance (param i32 i32)))
@@ -24,7 +24,12 @@
 
 	;; transfer 50 to CHARLIE
 	(func $call_2
-		(call $ext_transfer (i32.const 68) (i32.const 8) (i32.const 76) (i32.const 8))
+		(call $assert
+			(i32.eq
+				(call $ext_transfer (i32.const 68) (i32.const 8) (i32.const 76) (i32.const 8))
+				(i32.const 0)
+			)
+		)
 	)
 
 	;; do nothing