socket_http.c: Show curl error on failure

[kazarmy]

Your checklist for this pull request

• I've read the guidelines for contributing to this repository
• I made sure to follow the project's coding style
• I've documented or updated the documentation of every function and struct this PR changes. If not so I've explained why.
• I've added tests that prove my fix is effective or that my feature works (if possible)
• I've updated the rizin book with the relevant information (if needed)

Detailed description

This pr shows any errors generated by curl in socket_http.c, on stderr.

Test plan

For the Travis arm64 build, more info is shown regarding the pdb test failures. All other builds are green.

Closing issues

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