show proper error if input filename doesn't exist

src/error.py

@@ -7,3 +7,4 @@ class GPLTError(Enum):
     MODULE_SHOULD_NOT_RUN = 2
     COULD_NOT_DECODE_JSON = 3
     COULD_NOT_CONVERT = 4
+    NO_SUCH_FILE = 5

src/log.py

@@ -14,6 +14,8 @@ def panic(err: GPLTError, msg: str):
             x = "COULD_NOT_DECODE_JSON"
         case GPLTError.COULD_NOT_CONVERT:
             x = "COULD_NOT_CONVERT"
+        case GPLTError.NO_SUCH_FILE:
+            x = "NO_SUCH_FILE"
         case _:
             print(f"[bold red]PANIC[/bold red]: unexpected error: {err} is not known")
             exit(1)

src/main.py

@@ -16,6 +16,11 @@ VERSION = "{{VERSION}}"
 
 def load_json(json_value: str, filename: str = None):
     if filename is not None:
+        if not os.path.exists(filename):
+            panic(
+                GPLTError.NO_SUCH_FILE,
+                f"{filename}: no such file",
+            )
         info(f"loading JSON data from {filename}")
         with open(filename, "r") as file:
             try: