fix: Add constructor for McpError to allow setting field

Backport of #116 by @allenporter to v1.1.x branch.

Fixes an issue where exception handling code fails with AttributeError
when accessing the error field of McpError.

Author: allenporter

Diff for: src/mcp/shared/exceptions.py

@@ -7,3 +7,8 @@ class McpError(Exception):
     """
 
     error: ErrorData
+
+    def __init__(self, error: ErrorData):
+        """Initialize McpError."""
+        super().__init__(error.message)
+        self.error = error