Implement safe_get_content

[egeulgen]

some tests unexpectedly failed on CRAN checks:

══ Failed tests ════════════════════════════════════════════════════════════════
    ── Error ('test-data_generation.R:49:3'): `get_biogrid_pin()` -- determines and downloads the latest version ──
    <curl_error_operation_timedout/curl_error/error/condition>
    Error in `curl::curl_fetch_memory(url, handle = handle)`: Timeout was reached [downloads.thebiogrid.org]:
    Connection timed out after 10001 milliseconds
    Backtrace:
        ▆
     1. ├─pathfindR:::get_biogrid_pin() at test-data_generation.R:49:3
     2. │ └─httr::GET("https://downloads.thebiogrid.org/BioGRID/Latest-Release/")
     3. │   └─httr:::request_perform(req, hu$handle$handle)
     4. │     ├─httr:::request_fetch(req$output, req$url, handle)
     5. │     └─httr:::request_fetch.write_memory(req$output, req$url, handle)
     6. │       └─curl::curl_fetch_memory(url, handle = handle)
     7. └─curl:::raise_libcurl_error(...)

CRAN policy:

'Packages which use Internet resources should fail gracefully with an informative message
if the resource is not available or has changed (and not give a check warning nor error).'

Updates have been implemented so that:

• the new safe_get_content function is utilised so that GET and content call failures are handled more gracefully
• tests updated so that there is no direct access to actual URLs (instead these are mocked)