Unable to install esp8266 board in Arduino ide

```
Error downloading https://downloads.arduino.cc/packages/package_index.json
Error downloading https://github.com/esp8266/Arduino/releases/download/2.7.1/esp8266-2.7.1.zip
java.lang.RuntimeException: java.lang.Exception: Error downloading https://github.com/esp8266/Arduino/releases/download/2.7.1/esp8266-2.7.1.zip
at cc.arduino.contributions.packages.ui.ContributionManagerUI.lambda$onInstallPressed$2(ContributionManagerUI.java:175)
at java.lang.Thread.run(Thread.java:748)
Caused by: java.lang.Exception: Error downloading https://github.com/esp8266/Arduino/releases/download/2.7.1/esp8266-2.7.1.zip
at cc.arduino.contributions.DownloadableContributionsDownloader.download(DownloadableContributionsDownloader.java:149)
at cc.arduino.contributions.DownloadableContributionsDownloader.download(DownloadableContributionsDownloader.java:83)
at cc.arduino.contributions.DownloadableContributionsDownloader.download(DownloadableContributionsDownloader.java:60)
at cc.arduino.contributions.packages.ContributionInstaller.install(ContributionInstaller.java:105)
at cc.arduino.contributions.packages.ui.ContributionManagerUI.lambda$onInstallPressed$2(ContributionManagerUI.java:172)
... 1 more
Caused by: javax.net.ssl.SSLHandshakeException: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
at sun.security.ssl.Alerts.getSSLException(Alerts.java:192)
at sun.security.ssl.SSLSocketImpl.fatal(SSLSocketImpl.java:1946)
at sun.security.ssl.Handshaker.fatalSE(Handshaker.java:316)
at sun.security.ssl.Handshaker.fatalSE(Handshaker.java:310)
at sun.security.ssl.ClientHandshaker.serverCertificate(ClientHandshaker.java:1639)
at sun.security.ssl.ClientHandshaker.processMessage(ClientHandshaker.java:223)
at sun.security.ssl.Handshaker.processLoop(Handshaker.java:1037)
at sun.security.ssl.Handshaker.process_record(Handshaker.java:965)
at sun.security.ssl.SSLSocketImpl.readRecord(SSLSocketImpl.java:1064)
at sun.security.ssl.SSLSocketImpl.performInitialHandshake(SSLSocketImpl.java:1367)
at sun.security.ssl.SSLSocketImpl.startHandshake(SSLSocketImpl.java:1395)
at sun.security.ssl.SSLSocketImpl.startHandshake(SSLSocketImpl.java:1379)
at sun.net.www.protocol.https.HttpsClient.afterConnect(HttpsClient.java:559)
at sun.net.www.protocol.https.AbstractDelegateHttpsURLConnection.connect(AbstractDelegateHttpsURLConnection.java:185)
at sun.net.www.protocol.https.HttpsURLConnectionImpl.connect(HttpsURLConnectionImpl.java:162)
at cc.arduino.utils.network.HttpConnectionManager.makeConnection(HttpConnectionManager.java:150)
at cc.arduino.utils.network.HttpConnectionManager.makeConnection(HttpConnectionManager.java:106)
at cc.arduino.utils.network.FileDownloader.openConnectionAndFillTheFile(FileDownloader.java:234)
at cc.arduino.utils.network.FileDownloader.downloadFile(FileDownloader.java:182)
at cc.arduino.utils.network.FileDownloader.download(FileDownloader.java:129)
at cc.arduino.contributions.DownloadableContributionsDownloader.download(DownloadableContributionsDownloader.java:147)
... 5 more
Caused by: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
at sun.security.validator.PKIXValidator.doBuild(PKIXValidator.java:397)
at sun.security.validator.PKIXValidator.engineValidate(PKIXValidator.java:302)
at sun.security.validator.Validator.validate(Validator.java:262)
at sun.security.ssl.X509TrustManagerImpl.validate(X509TrustManagerImpl.java:324)
at sun.security.ssl.X509TrustManagerImpl.checkTrusted(X509TrustManagerImpl.java:229)
at sun.security.ssl.X509TrustManagerImpl.checkServerTrusted(X509TrustManagerImpl.java:124)
at sun.security.ssl.ClientHandshaker.serverCertificate(ClientHandshaker.java:1621)
... 21 more
```

Start with cc.arduino.utils.network.HttpConnectionManager and FileDownloader, then trace the package installation path through DownloadableContributionsDownloader. Reproduce the esp8266 download failure and determine why HTTPS certificate validation fails for the package index and release archive; done means the installation succeeds without the PKIX path-building error.

Written by the indexing model from the issue text.