fix: library loading errors genereates unneeded compile failures

[cmaglie]

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What kind of change does this PR introduce?

• The library loading errors do not affect compile results anymore.
• Fixed an error message
• Library loading warnings are reported only in verbose compile

What is the current behavior?

If an invalid library is installed in the system, all sketch compilations will fail, even if the library is not used:

$ arduino-cli compile -b arduino:avr:uno
Error initializing instance: Loading libraries: &{0xc001c760c0}


Used platform Version Path                                                        
arduino:avr   1.8.5   /home/megabug/.arduino15/packages/arduino/hardware/avr/1.8.5
Error during build: rpc error: code = Internal desc = loading library from /home/megabug/Workspace/sketchbook-cores-beta/libraries/Empty: invalid library: no header files found

This behavior has been worsened after merging #2083 which caused more libraries to be reported as invalid.

What is the new behavior?

Any invalid library is silently ignored unless the verbose output is enabled. In that case, a warning is printed explaining the error:

$ arduino-cli compile -b arduino:avr:uno
Sketch uses 444 bytes (1%) of program storage space. Maximum is 32256 bytes.
Global variables use 9 bytes (0%) of dynamic memory, leaving 2039 bytes for local variables. Maximum is 2048 bytes.

Used platform Version Path                                                        
arduino:avr   1.8.5   /home/megabug/.arduino15/packages/arduino/hardware/avr/1.8.5

$ arduino-cli compile -b arduino:avr:uno -v
FQBN: arduino:avr:uno
Using board 'uno' from platform in folder: /home/megabug/.arduino15/packages/arduino/hardware/avr/1.8.5
Using core 'arduino' from platform in folder: /home/megabug/.arduino15/packages/arduino/hardware/avr/1.8.5

loading library from /home/megabug/Workspace/sketchbook-cores-beta/libraries/Empty: invalid library: no header files found
Detecting libraries used...
/home/megabug/.arduino15/packages/arduino/tools/avr-gcc/7.3.0-atmel3.6.1-arduino7/bin/avr-g++ -c -g -Os -w -std=gnu++11 -fpermissive -fno-exceptions -ffunction-sections -fdata-sections -fno-threadsafe-statics -Wno-error=narrowing -flto -w -x c++ -E -CC -mmcu=atmega328p -DF_CPU=16000000L -DARDUINO=10607 -DARDUINO_AVR_UNO -DARDUINO_ARCH_AVR -I/home/megabug/.arduino15/packages/arduino/hardware/avr/1.8.5/cores/arduino -I/home/megabug/.arduino15/packages/arduino/hardware/avr/1.8.5/variants/standard /tmp/arduino/sketches/002050EAA7EFB9A4FC451CDFBC0FA2D3/sketch/Blink.ino.cpp -o /dev/null
Using cached library dependencies for file: /tmp/arduino/sketches/002050EAA7EFB9A4FC451CDFBC0FA2D3/sketch/Blink.cpp
Generating function prototypes...
/home/megabug/.arduino15/packages/arduino/tools/avr-gcc/7.3.0-atmel3.6.1-arduino7/bin/avr-g++ -c -g -Os -w -std=gnu++11 -fpermissive -fno-exceptions -ffunction-sections -fdata-sections -fno-threadsafe-statics -Wno-error=narrowing -flto -w -x c++ -E -CC -mmcu=atmega328p -DF_CPU=16000000L -DARDUINO=10607 -DARDUINO_AVR_UNO -DARDUINO_ARCH_AVR -I/home/megabug/.arduino15/packages/arduino/hardware/avr/1.8.5/cores/arduino -I/home/megabug/.arduino15/packages/arduino/hardware/avr/1.8.5/variants/standard /tmp/arduino/sketches/002050EAA7EFB9A4FC451CDFBC0FA2D3/sketch/Blink.ino.cpp -o /tmp/arduino/sketches/002050EAA7EFB9A4FC451CDFBC0FA2D3/preproc/ctags_target_for_gcc_minus_e.cpp

Does this PR introduce a breaking change, and is titled accordingly?

Other information

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01 🎉

Comparison is base (fe91ec6) 62.59% compared to head (13249e1) 62.60%.

Additional details and impacted files

@@            Coverage Diff             @@
##           0.32.x    #2139      +/-   ##
==========================================
+ Coverage   62.59%   62.60%   +0.01%     
==========================================
  Files         227      227              
  Lines       19533    19534       +1     
==========================================
+ Hits        12226    12230       +4     
  Misses       6210     6210              
+ Partials     1097     1094       -3     

Flag	Coverage Δ
unit	62.60% <100.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
commands/instances.go	58.35% <100.00%> (ø)
legacy/builder/libraries_loader.go	55.00% <100.00%> (+16.01%)	⬆️

... and 1 file with indirect coverage changes

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