Output user friendly message when platform installation from CLI fails due to missing package index entry

```
arduino --install-boards esp8266:esp8266
Picked up JAVA_TOOL_OPTIONS:
Loading configuration...
Initializing packages...
Preparing boards...
Downloading platforms index...
Downloading platforms index... Downloaded 0kb of 197kb.
Downloading platforms index... Downloaded 10kb of 197kb.
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Downloading platforms index... Downloaded 0kb of 0kb.
Downloading platforms index... Downloaded 0kb of 0kb.
java.lang.NullPointerException
at java.util.Collections.sort(Collections.java:175)
at processing.app.Base.(Base.java:310)
at processing.app.Base.main(Base.java:144)
make: *** [deps] Error 255
```

This running on [travis](https://travis-ci.org/IBEXcps/HaLiO-sw/builds/310596858).

Reproduce the failure with `arduino --install-boards esp8266:esp8266`, then inspect `processing.app.Base` at the constructor and `Base.main` locations shown in the stack trace (`Base.java:310` and `Base.java:144`). The work is done when a missing package-index entry produces a user-friendly CLI error instead of a `NullPointerException`.

Written by the indexing model from the issue text.