make error reporting on signature loading clearer

[ctb]

For now, load_signatures will by default print out an error message when it cannot load a signature from a file, but will not raise an exception. The error message isn't particularly clear - at the least we should make sure to report what file is causing the problem.

A complementary and perhaps better idea is to write a new function for loading signatures from multiple files and directories that does better reporting

[ctb]

(see the machinations needed in the implementation of sourmash sig describe to accurately report errors for an example of this :)

[ctb]

hey, cool, this was fixed in #1059