better error messages/handling for Samples indexing

[jrevels]

@SimonDanisch ran into some issues - Simon please post them here when you get a chance :D

EDIT: this is borderline superseded by #77

[SimonDanisch]

using Onda, OndaViz, Dates

# Use your favorite way to procure some samples:
viewer = OndaViz.OndaViewer();
eeg = viewer.signal_state.signals[][:eeg]::Samples;
# Error I got when I forgot, that one needs to use TimeSpan
eeg[1:2, Nanosecond(1):Nanosecond(45)]
# > MethodError: Cannot `convert` an object of type Int64 to an object of type Nanosecond

# Error if you index with a non existing channel symbol:
eeg[:f1, 1]
# > ArgumentError: invalid index: nothing of type Nothing

[jrevels]

Nanosecond(1):Nanosecond(45)
MethodError: Cannot convert an object of type Int64 to an object of type Nanosecond

This one is a Base problem, or at least it can't really be fixed without type piracy

eeg[:f1, 1]
ArgumentError: invalid index: nothing of type Nothing

this one should be better though lol

[kleinschmidt]

still bumping into this.