Mention log file if an error happened before building, too

[native-api]

Make sure you have checked all steps below.

Prerequisite

• Please consider implementing the feature as a hook script or plugin as a first step.
  • pyenv has some powerful support for plugins and hook scripts. Please refer to Authoring plugins for details and try to implement it as a plugin if possible.
    N/A -- bugfix
• Please consider contributing the patch upstream to rbenv, since we have borrowed most of the code from that project.
  • We occasionally import the changes from rbenv. In general, you can expect changes made in rbenv will be imported to pyenv too, eventually.
  • Generally speaking, we prefer not to make changes in the core in order to keep compatibility with rbenv.
    N/A -- the code in question is not present in rbenv
• My PR addresses the following pyenv issue (if any)
  • pyenv install fails silently when downloader not found #1156

Description

• Here are some details about my PR
  When a build fails, mention log file and its contents regardless of whether $BUILD_PATH exists

Tests

• My PR adds the following unit tests (if any)
  The current tests seem to only be testing public functionality while build_failed() is private. Should I add a test for it? Is it okay to test it directly? If not, which endpoint should I test it via?