Add pillow to the project's dependencies.

Also fix the relevant error message's formatting.

pyproject.toml

@@ -16,6 +16,8 @@ dependencies = [
     "omgifol (>=0.5.1,<0.6.0)",
     "tomlkit (>=0.13.3,<0.14.0)",
     "wand (>=0.6.13,<0.7.0)",
+    "pytest (>=8.4.2,<9.0.0)",
+    "pillow (>=11.3.0,<12.0.0)",
 ]
 
 
@@ -31,3 +33,8 @@ dcc = { source = "dcc/main.py", type = "onefile" }
 
 [tool.poetry-pyinstaller-plugin.collect]
 all = ["cliff"]
+
+[tool.pytest.ini_options]
+testpaths = [
+	"tests",
+]