Show path in the error message (#1396)

[u7693]

With this pull request, in the case of the example in #1396, the error message will look like this:

2020-12-14 15:31:55 [INFO] (mdbook::book): Book building has started
2020-12-14 15:31:55 [INFO] (mdbook::book): Running the html backend
2020-12-14 15:31:55 [ERROR] (mdbook::utils): Error: Rendering failed
2020-12-14 15:31:55 [ERROR] (mdbook::utils):    Caused By: Failed to read "/home/u7693/Workspace/book/src/not-found"
2020-12-14 15:31:55 [ERROR] (mdbook::utils):    Caused By: No such file or directory (os error 2)

Fixes #1396

[ehuss]

Thanks!