Add error handling

Signed-off-by: Glenn Lewis <[email protected]>

README.md

@@ -76,8 +76,8 @@ Now paste this into your `main/main.mbt` file:
 
 ```rust
 pub fn greet() -> Int {
-  let input = @host.input_string()
-  let greeting = "Hello, \(input)!"
+  let name = @host.input_string()
+  let greeting = "Hello, \(name)!"
   @host.output_string(greeting)
   0 // success
 }
@@ -130,6 +130,38 @@ extism call target/wasm/release/build/main/main.wasm greet --input "Benjamin" --
 
 > **Note**: We also have a web-based, plug-in tester called the [Extism Playground](https://playground.extism.org/)
 
+### More Exports: Error Handling
+
+Suppose we want to re-write our greeting module to never greet Benjamins.
+We can use [`@host.set_error`](https://mooncakes.io/docs/#/extism/moonbit-pdk/pdk/host/members?id=set_error):
+
+```rust
+pub fn greet() -> Int {
+  let name = @host.input_string()
+  if name == "Benjamin" {
+    @host.set_error("Sorry, we don't greet Benjamins!")
+    return 1 // failure
+  }
+  let greeting = "Hello, \(name)!"
+  @host.output_string(greeting)
+  0 // success
+}
+```
+
+Now when we try again:
+
+```bash
+moon build --target wasm
+extism call target/wasm/release/build/main/main.wasm greet --input "Benjamin" --wasi
+# => Error: Sorry, we don't greet Benjamins!
+echo $? # print last status code
+# => 1
+extism call target/wasm/release/build/main/main.wasm greet --input "Zach" --wasi
+# => Hello, Zach!
+echo $?
+# => 0
+```
+
 ## For PDK Devs: Building the PDK locally
 
 Before building, you must have already installed the MoonBit programming language,

moon.mod.json

@@ -1,6 +1,6 @@
 {
   "name": "extism/moonbit-pdk",
-  "version": "0.16.0",
+  "version": "0.17.0",
   "deps": {
     "gmlewis/json": "0.11.0"
   },

pdk/host/host.mbt

@@ -42,6 +42,18 @@ pub fn output_json_value(j : @json.JsonValue) -> Unit {
   @jsonutil.stringify(j, spaces=0, newline=false) |> output_string()
 }
 
+fn set_error_bytes(b : Bytes) -> Unit {
+  let mem = output_bytes_to_memory(b)
+  @extism.error_set(mem.offset)
+  mem.free()
+}
+
+/// `set_error` converts a MoonBit String (UTF-16) to an Extism string (UTF-8)
+/// and sends it to the host as its error output.
+pub fn set_error(s : String) -> Unit {
+  @pdk.ToUtf8::to_utf8(s) |> set_error_bytes()
+}
+
 /// `log_warn_str` is a helper function to log a warn string to the host.
 pub fn log_warn_str(s : String) -> Unit {
   let { offset, .. } = @pdk.ToUtf8::to_utf8(s) |> output_bytes_to_memory()