dependencies/pkgconfig: Log pkg-config exceptions
dependencies/pkgconfig: Log pkg-config exceptions

This means that when either cflags or libs lookup fails, this will be
logged always in the log, leading to much easier to debug messages
like:

  Called `/usr/local/bin/pkg-config --cflags gnutls` -> 1

  pkg-config error with 'gnutls': Could not generate cargs for gnutls:
  Package nettle was not found in the pkg-config search path.
  Perhaps you should add the directory containing `nettle.pc'
  to the PKG_CONFIG_PATH environment variable
  Package 'nettle', required by 'gnutls', not found

  Run-time dependency gnutls found: NO (tried pkgconfig)

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