Improve error reporting when source cannot be minimized (#16)

* Improve error reporting when source cannot be minimized

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

pysource_minimize/__init__.py

@@ -1,8 +1,9 @@
+from ._minimize import CouldNotMinimize
 from ._minimize import minimize
 from ._minimize_base import StopMinimization
 
 
-__all__ = ("minimize", "StopMinimization")
+__all__ = ("minimize", "CouldNotMinimize", "StopMinimization")
 
 
 version = "0.6.2"

pysource_minimize/_minimize.py

@@ -54,6 +54,10 @@ def minimize_ast(
     return current_ast
 
 
+class CouldNotMinimize(ValueError):
+    """Raised to indicate that the source code could not be minimized."""
+
+
 def minimize(
     source: str,
     checker: Callable[[str], bool],
@@ -87,7 +91,10 @@ def source_checker(new_ast):
         return checker(source)
 
     if not source_checker(original_ast):
-        raise ValueError("ast.unparse removes the error minimize can not help here")
+        raise CouldNotMinimize(
+            "Source code cannot be minimized: the error failed to reproduce "
+            "after roundtripping the source using `ast.parse()` and `ast.unparse()`"
+        )
 
     minimized_ast = minimize_ast(
         original_ast,