Add a useful error message instead of "[!] send_recv"

Don't read the target voltage on startup, because it crashes STM32F100.

WIP: Fix compiler warnings

 - Replace 'typedef reg' by 'struct stlink_reg'
 - Prefix the 'sl' global variable with a 'g'
 - Rename 'j' -> 'k' and 'i' -> 'j' variables
 - Rename 'hex' variable into 'hextmp'
 - Fix unused parameter
 - Use a correct printf format for gsize
 - Use correct MAX macro
 - Use correct data casting
 - Do not use 'GTK_STOCK_*' anymore
 - Do not use 'gtk_tree_view_set_rules_hint'
 - Remove unused functions
 - Explicitly cast data type
 - Enable Werror

Related to https://github.com/texane/stlink/issues/356

* 'master' of github.com:texane/stlink:
  travis: Add gcc-5 and clang-3.8 to matrix for linux cmake builds

travis: Add gcc-5 and clang-3.8 to matrix for linux cmake builds

cmake: Search for static libusb-1.0 library on OSX and link with ObjC,CoreFoundation and IOKit frameworks

logging.h: Add helpers so the log macros can have just one argument, needed for compilation on visual studio

Fix const warning on struct stlink_chipid_params

Fix link to UweBonnes/wiki_fuer_alex

Add Contributing and versioning to README

stlink_gui: Fix compiler warning on overflow

Move chipid defines and deduplicate chipid params code

Add L0 Category 2 device (chip id: 0x425), fixes #380 

Reorganize stlink-gui and update README, add building gui with Travis

Fixing and merging of STM32L0 and STM32L1 loaders