python: raise OSError when shared library is missing

Replace sys.exit(1) in WeaselJsonParser.__init__ with an OSError so
callers can handle a missing libweaseljson gracefully. Also add a test
that verifies the constructor raises OSError for a non-existent build
directory.

Closes #38

test_python_bindings.py

@@ -95,8 +95,20 @@ def test_create_rejects_too_small_stack():
         raise AssertionError(f"expected ValueError for stackSize={stack_size}")
 
 
+def test_missing_library_raises_oserror():
+    try:
+        weaseljson.WeaselJsonParser(
+            weaseljson.WeaselJsonCallbacksBase(),
+            build_dir="/nonexistent",
+        )
+    except OSError:
+        return
+    raise AssertionError("expected OSError when the shared library is missing")
+
+
 if __name__ == "__main__":
     test_object_keys_routed_correctly()
     test_mixed_values()
     test_create_rejects_too_small_stack()
+    test_missing_library_raises_oserror()
     print("python bindings ok")

weaseljson.py

@@ -84,13 +84,7 @@ class WeaselJsonParser:
                 pass
 
         if self._lib is None:
-            import sys
+            raise OSError(f"Could not load libweaseljson from {build_dir}")
-
-            print(
-                "Could not find libweaseljson implementation",
-                file=sys.stderr,
-            )
-            sys.exit(1)
 
         self._lib.WeaselJsonParser_create.argtypes = (
             ctypes.c_int,