Change error message for IndexError for increased clarity

[ECP5-PCIe]

If you for example have a signal x = Signal(16) and try to address a bit which is out of bounds, for example x[16] you get an IndexError: Cannot index 16 bits into 16-bit value.
This is somewhat unclear, since 16 bits sounds like it refers to bit number 16, which this signal has. But in reality it refers to the addressed index.
I suggest renaming it to something like IndexError: Index 16 is out of bounds for a Signal of length 16 such that it is clear that 16 refers to an index (which start at 0) and not to a bit.