Rephrase more errors and warnings

[MaEtUgR]

Describe problem solved by this pull request
Porting @DanielePettenuzzo 's changes, credit goes to him.

Describe your solution
More error rephrasing. These are the ones done in this pr:

• change kill switch from error to info when the vehicle is not flying
• "Data link lost" --> "Connection to ground station lost"
• "Onboard controller lost" --> "Connection to mission computer lost"
• "Failsafe enabled: no RC" --> "Failsafe enabled: Manual control stick input unavailable"

Additional context
EDIT: just found a related non-conflicting pr changing some messageing: #14377

[bresch]

I'm fine with the rephrasing, but this adds quite a few chars. Could we save the strings "Manual kill-switch" and "engaged", "disengaged" in variables to re-use them after? Also. isn't "Kill-switch engaged" enough? I'm not sure if the "manual" is really useful here. If "manual" is really required, we should rename _manual_control_setpoint.kill_switch...

[MaEtUgR]

I saved 53 unecessary chars and fixed the build with adaption to #14848.
@bresch what do you think? The messages "engaged" and "disengaged" could be saved in parts but the assembly takes more code... I don't consider it worth it. I imagined something along constexpr char[] KILL_SWITCH = "Kill-switch"; and on the stack

char output[50];
snprintf(output, 50, "", "%s %s", KILL_SWITCH, ENGAGED);

[Antiheavy]

Nice improvements!