feat: Proper error handling

- Bump version to 0.8.0
- Added discord error struct
- All errors now implement std::error::Error
- Implemented error handler instead of relying on default
- Fixed bug where you could send answer on a completed quest

Cargo.lock

@@ -1599,7 +1599,7 @@ dependencies = [
 
 [[package]]
 name = "squad-quest"
-version = "0.7.0"
+version = "0.8.0"
 dependencies = [
  "serde",
  "toml",
@@ -1607,7 +1607,7 @@ dependencies = [
 
 [[package]]
 name = "squad-quest-cli"
-version = "0.7.0"
+version = "0.8.0"
 dependencies = [
  "chrono",
  "clap",
@@ -1618,7 +1618,7 @@ dependencies = [
 
 [[package]]
 name = "squad-quest-discord"
-version = "0.7.0"
+version = "0.8.0"
 dependencies = [
  "clap",
  "dotenvy",