Propagate exceptions fully when using proc_lib

This makes proc_lib behaves like a normal process as far
as the propagation of exceptions is concerned.

Before this commit, the following difference could be
observed:

  6> spawn_link(fun() -> ssl:send(a,b) end).
  <0.43.0>
  7> flush().
  Shell got {'EXIT',<0.43.0>,
             {function_clause,
              [{ssl,send,[a,b],[{file,"..."},{line,275}]}]}}
  ok

  8> proc_lib:spawn_link(fun() -> ssl:send(a,b) end).
  <0.46.0>
  9> flush().
  Shell got {'EXIT',<0.46.0>,function_clause}

After this commit, we get the following instead:

  3> flush().
  Shell got {'EXIT',<0.61.0>,
             {function_clause,
              [{ssl,send,[a,b],[{file,"..."},{line,275}]},
               {proc_lib,init_p,3,[{file,"..."},{line,232}]}]}}

The stacktrace will show minor differences of course
but the form is now the same as without proc_lib.

The rationale behind this commit is that:

* We now have a single form regardless of how the process
  was started

* We can use the stacktrace to programmatically alter behavior
  (for example an HTTP server identifying problems in input
  decoding to send back a generic 400, or a 500 otherwise)

* We can access the stacktrace to print it somewhere (for
  example an HTTP server could send it back to the client
  when a debug mode is enabled)

1 files changed, 6 insertions, 0 deletions

diff --git a/lib/stdlib/doc/src/proc_lib.xml b/lib/stdlib/doc/src/proc_lib.xml
index da03c39a26..e64b2ce18a 100644
--- a/lib/stdlib/doc/src/proc_lib.xml
+++ b/lib/stdlib/doc/src/proc_lib.xml
@@ -66,6 +66,12 @@
       <seealso marker="sasl:error_logging">SASL Error Logging</seealso>
       in the SASL User's Guide.</p>
 
+    <p>Unlike in "plain Erlang", <c>proc_lib</c> processes will not generate
+      <em>error reports</em>, which are written to the terminal by the
+      emulator and do not require SASL to be started. All exceptions are
+      converted to <em>exits</em> which are ignored by the default
+      <c>error_logger</c> handler.</p>
+
     <p>The crash report contains the previously stored information, such
       as ancestors and initial function, the termination reason, and
       information about other processes that terminate as a result