Added error messages for RH_RF95 init

[scottbarnesg]

Fixes Issue #31: Insufficient error messages on RH_RF95 init.

Adds error messages for each failure (return false) condition of the RH_RF95 init function. This will make debugging initialization issues significantly easier for users.

[ladyada]

Serial.println()'ing from a library is considered bad taste - you may want to, instead, have an #ifdef that people can turn on for more debugging - that's a much better ideas! also please use F("string") so AVRs dont use RAM for strings. thanks!

[scottbarnesg]

Updated per @ladyada's feedback:

• added DEBUG_SERIAL definition (commented out)
• added ifdef around all Serial.print statements
• added F("string") for eligible Serial.print statements (exception: line 74 not defined until run time).
• added print statement in rf95 failure condition for feather rf95 examples that use RH_RF95 class that describes how to enable debug mode.

Another option would be to update the RH_RF95 class with optional debug parameter that defaults to false, but I wasn't sure if this would cause compilation issues with environments that don't support Serial. Are there any target environments that don't support Serial? If not, I think this would be the better option.

[ladyada]

nice thank you :)