Better error message when trying to replace CallExpressions instead of ExpressionStatement or VariableDeclarator

I hit a vague "Could not replace path" error today: http://astexplorer.net/#/gist/cfb2f93a39c4a80a1ecf44930092b59f/edca9a554a9aad19c62f098b07ee024a9c418241

After digging around for a while, I found #157, where @fkling wrote:

> You should not be looking for CallExpressions but for ExpressionStatements that contain a CallExpression that matches your constraints.

This was exactly my issue! Changing the script to instead look for the `ExpressionStatement` worked fine: http://astexplorer.net/#/gist/cfb2f93a39c4a80a1ecf44930092b59f/7b4a6f22da67d31cb18e768e0da975f26244ea1e

Can we make the "Could not replace path" error better?