Introduce errors submodule

Lib/fontTools/varLib/errors.py

@@ -0,0 +1,39 @@
+class VarLibError(Exception):
+    """Base exception for the varLib module."""
+
+
+class VarLibValidationError(VarLibError):
+    """Raised when input data is invalid from varLib's point of view."""
+
+
+class VarLibMergeError(VarLibError):
+    """Raised when input data cannot be merged into a variable font."""
+
+
+class VarLibCFFDictMergeError(VarLibMergeError):
+    """Raised when a CFF PrivateDict cannot be merged."""
+
+    def __init__(self, key, value, values):
+        error_msg = (
+            f"For the Private Dict key '{key}', the default font value list:"
+            f"\n\t{value}\nhad a different number of values than a region font:"
+        )
+        for region_value in values:
+            error_msg += f"\n\t{region_value}"
+        self.args = (error_msg,)
+
+
+class VarLibCFFPointTypeMergeError(VarLibMergeError):
+    """Raised when a CFF glyph cannot be merged."""
+
+    def __init__(self, point_type, pt_index, m_index, default_type, glyph_name):
+        error_msg = (
+            f"Glyph '{glyph_name}': '{point_type}' at point index {pt_index} in "
+            f"master index {m_index} differs from the default font point type "
+            f"'{default_type}'"
+        )
+        self.args = (error_msg,)
+
+
+class VariationModelError(VarLibError):
+    """Raised when a variation model is faulty."""