Improve error handling about bh1750

[coelner]

Maybe there is another flaw: If we use H-Resolution Mode2 we get a 0.5 resolution. Therefore we need a float as return value. And in that case we could use negative numbers

[bjung82]

I added an else-path at BH1750.cpp:183
with

Wire.reset();

Without this the I2C bus was held down forever. It took me some hours
because the problem was not easily reproducible.

Here is the PR
#41

[coelner]

@bjung82 explanation @ #41

[coelner]

Any issue left or should this topic be closed?