Export exception classes with default visibility

Summary:
This fixes a crash caused by a `JSError` exception not being successfully caught in a different dynamic library from where it was thrown. Since the libraries were compiled with hidden symbols and loaded with `RTLD_LOCAL`, the exception typeinfo becomes unique to each module.

Reading on this subject:
https://gcc.gnu.org/wiki/Visibility
https://stackoverflow.com/questions/14268736/symbol-visibility-exceptions-runtime-error

Reviewed By: mhorowitz

Differential Revision: D19343161

fbshipit-source-id: 4eb3bc2576bbcca2c3aef4f52b5a27dfde214c6a

ReactCommon/jsi/jsi/jsi.h

@@ -1191,7 +1191,7 @@ class Scope {
 };
 
 /// Base class for jsi exceptions
-class JSIException : public std::exception {
+class JSI_EXPORT JSIException : public std::exception {
  protected:
   JSIException(){};
   JSIException(std::string what) : what_(std::move(what)){};
@@ -1207,15 +1207,15 @@ class JSIException : public std::exception {
 
 /// This exception will be thrown by API functions on errors not related to
 /// JavaScript execution.
-class JSINativeException : public JSIException {
+class JSI_EXPORT JSINativeException : public JSIException {
  public:
   JSINativeException(std::string what) : JSIException(std::move(what)) {}
 };
 
 /// This exception will be thrown by API functions whenever a JS
 /// operation causes an exception as described by the spec, or as
 /// otherwise described.
-class JSError : public JSIException {
+class JSI_EXPORT JSError : public JSIException {
  public:
   /// Creates a JSError referring to provided \c value
   JSError(Runtime& r, Value&& value);