Fix catchall exceptions.

Using ValueError for everything caused underlying issues to be
hidden. Now there are explicit classes for each error condition.

test/test_rewrite.py

@@ -77,8 +77,8 @@ def test_error_bad_path():
         (project.dir / "setup.py").unlink()
         try:
             list(rewrite._iter_file_paths(cfg.file_patterns))
-            assert False, "expected ValueError"
-        except ValueError as ex:
+            assert False, "expected FileNotFoundError"
+        except FileNotFoundError as ex:
             assert "setup.py" in str(ex)
 
 
@@ -93,6 +93,5 @@ def test_error_bad_pattern():
 
         try:
             list(rewrite.diff("v201809.1234", patterns))
-            assert False, "expected ValueError"
-        except ValueError as ex:
+        except rewrite.NoPatternMatch as ex:
             assert "setup.py" in str(ex)

test/test_version.py

@@ -107,7 +107,7 @@ def test_parse_error_empty():
     try:
         version.parse_version_info("")
         assert False
-    except ValueError as err:
+    except version.PatternError as err:
         pass
 
 
@@ -115,7 +115,7 @@ def test_parse_error_noprefix():
     try:
         version.parse_version_info("201809.0002")
         assert False
-    except ValueError as err:
+    except version.PatternError as err:
         pass
 
 
@@ -123,7 +123,7 @@ def test_parse_error_nopadding():
     try:
         version.parse_version_info("v201809.2b0")
         assert False
-    except ValueError as err:
+    except version.PatternError as err:
         pass