add missing exceptions

exceptions.py

@@ -0,0 +1,32 @@
+# encoding: utf-8
+
+"""
+Exceptions used with python-docx.
+The base exception class is PythonDocxError.
+"""
+
+
+class PythonDocxError(Exception):
+    """
+    Generic error class.
+    """
+
+
+class PendingDeprecationWarning(PythonDocxError):
+    """
+    Raised when a tool pending deprecation is used.
+    """
+
+
+class InvalidSpanError(PythonDocxError):
+    """
+    Raised when an invalid merge region is specified in a request to merge
+    table cells.
+    """
+
+
+class InvalidXmlError(PythonDocxError):
+    """
+    Raised when invalid XML is encountered, such as on attempt to access a
+    missing required child element
+    """