Path not found

[vch9]

If the path directory/path given is not found, Osnap raises:
(Sys_error ".osnap/exponentiation: No such file or directory").

We should either:

• Give the real absent directory (e.g. pwd/.osnap/)
• Create the directory? (related to Allows optional path #11)

[vch9]

Actually both should be done:

• W/R errors should display the full path: $pwd/.osnap/[test_name]
• Write a snapshot should create the file and directory (therefore, fix Allows optional path #11 first)

[vch9]

• Creation of directory/file was done in e54bdd1
• W/R errors are not much better handled. At least when runner mode is Error, the path is fully displayed when the precedent snapshot does not exist.