Print error message on certificate flash failure

[per1234]

Previously, if there was a problem during the flashing stage of the certificate flash command invocation, Arduino Firmware Uploader would exit silently, giving the user no feedback that something had gone wrong other than a generic exit status 1:

$ touch /tmp/cert-with-unsupported-extension.crt

$ arduino-fwuploader certificates flash --address COM42 --file /tmp/cert-with-unsupported-extension.crt --fqbn arduino:renesas_uno:unor4wifi

Converting and flashing certificate C:/Users/per/AppData/Local/Temp/cert-with-unsupported-extension.crt

$ echo $?

1

After the change proposed here, Arduino Firmware Updater clearly communicates to the user that the command failed, and why:

$ arduino-fwuploader certificates flash --address COM42 --file /tmp/cert-with-unsupported-extension.crt --fqbn arduino:renesas_uno:unor4wifi

Converting and flashing certificate C:/Users/per/AppData/Local/Temp/cert-with-unsupported-extension.crt
Error during certificates flashing: cert format .crt not supported, please use .pem or .cer

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (45b115d) 12.10% compared to head (9c1e4eb) 12.10%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #215   +/-   ##
=======================================
  Coverage   12.10%   12.10%           
=======================================
  Files          20       20           
  Lines        1165     1165           
=======================================
  Hits          141      141           
  Misses        971      971           
  Partials       53       53           

Flag	Coverage Δ
unit	12.10% <0.00%> (ø)

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Files Changed	Coverage Δ
cli/certificates/flash.go	0.00% <0.00%> (ø)

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[alessio-perugini]

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