Implement xpath error message handling

[hurricane-socrates]

See #133 for context.

Two issues so far:

1. The first test isn't correct yet. I wanted to start this PR.
2. After each source change I have to redefine LIBXML2. I'm not sure how to tell Cargo where the correct libxml2 resides. I think it's finding the Apple version, not the brew version. There's also a MacPorts version that might be in the search path.

[anwaralameddin]

On macOS, pkg-config-0.3.2 is used in the build script to locate libxml2 when LIBXML2 is not set. Setting the atleast_version and replacing the function find_library with the method find might help, but I have not tested it. For local tests, it might be simpler to reorder the entries in the library search paths.

Anyway, this breaks tests for anyone using a released version of libxml2 regardless of OS. Can the PR objective be achieved using xmlSetStructuredErrorFunc instead of xmlXPathSetErrorHandler? If so, you could consider using it until version 2.13.0 is resealed and widely adopted. For reference, ubuntu's latest release still uses libxml2 version 2.9.14. Otherwise, if xmlSetStructuredErrorFunc does not work here, you could consider conditional compilation on the libxml2 version.

[hurricane-socrates]

Yeah, I was going around on the question of version support. I have no problem waiting. I see Debian Stable is also on the 2.9 branch

I couldn't get the build script to work with the updated HEAD version. I can see it uses pkg-config, and brew creates a correct definition. Nevertheless... I'll try pounding on it some more. It's probably got something to do with brew not symlinking libxml2 b/c Apple

In the meantime, the whole point of this task for me was that libxml2 reported line & col. It has stopped doing that reporting. Very annoying.

I'll look at xmlSetStructuredErrorFunc. It doesn't ring a bell right now. I'm sure it'll be as useful as xmlXPathSetErrorHandler has turned out to be. Right now I am annoyed at developing OSS on macos. At least three versions of libxml: Apple's, MacPort's, Brew's.

[hurricane-socrates]

I'd like to withdraw this PR.

0. Implementing xmlXPathSetErrorHandler for this task increases this project's technical debt;
1. libxml has stopped reporting detailed xpath errors. I'll look into why this is happening. The change nullifies my reasons for using xmlSetStructuredErrorFunc.

[dginev]

Thank you for investigating this @hurricane-socrates and feel free to close here if you've chosen to do so.